Physics & Physiology
Dive Physics
Why water behaves so differently from air — density, pressure, buoyancy, heat loss, light refraction and colour loss, and sound underwater, all explained for divers.
Water resembles air in one respect: it is a fluid medium you move through. Physically, however, it is almost the opposite. Water is roughly 800 times denser than air, conducts heat about 25 times faster, bends and absorbs light differently, and carries sound more than four times faster. Each of these differences changes something fundamental about how you function underwater.
A diver who understands the physics is surprised by nothing. You understand why an object looks larger and closer underwater, why you can still get cold in tropical water, why you struggle to tell which direction a boat is coming from, and why you need less lead with a steel tank than with an aluminium one. This guide treats these phenomena not as isolated facts but from their causes. Once you know the cause, you can work out the consequences yourself, even in situations no book describes precisely.
What "physics" means in practice here. You do not need to become a physicist. But a divemaster who has to explain to a student why a fish in their mask looks 33% bigger, or why their buddy started shivering in 22°C water, needs more than "that's just how it is". The physics below is exactly what you need to answer such questions calmly and correctly.
Key concepts
| Term | Short definition | Why it matters |
|---|---|---|
| Density | Mass per volume (kg/m³) | Governs buoyancy, drag and heat conduction |
| Fresh vs. salt water | Salt water is denser (~1025 vs. ~1000 kg/m³) | You float better in the sea; you need more lead |
| Hydrostatic pressure | Pressure from the weight of the water column above you | Rises ~1 bar per 10 m; the engine behind all pressure effects |
| Absolute vs. gauge pressure | Absolute includes the atmosphere; gauge is above it | Prevents errors in gas and depth calculations |
| Buoyant force (Archimedes) | Upward force = weight of displaced water | Explains sinking, hovering and rising |
| Thermal conduction | Rate at which heat drains away | Water pulls heat ~25x faster than air |
| Refraction | Bending of light at the water/air boundary | Objects look ~33% bigger and ~25% closer |
| Light absorption | Water absorbs colour, red first | Why deep images look blue-green |
| Speed of sound in water | ~1500 m/s, over 4x faster than in air | You hear well, but judge direction poorly |
Understanding the physics
Density: the property that drives everything
Density is mass per volume. Water is about 800 times denser than air at sea level. Almost every difference between "life above water" and "life underwater" comes back to that single property: because water is so dense, a column of it weighs a lot (hence the fast pressure rise); your body displaces a substantial mass (hence buoyancy); the molecules are packed close and conduct heat away quickly (hence hypothermia); and a vibration travels fast (hence the high speed of sound).
Fresh versus salt. Seawater contains dissolved salts and is therefore denser (about 1025 kg/m³) than fresh water (about 1000 kg/m³). That ~2.5% difference sounds small, but it means you have more buoyancy in the sea and therefore need more lead than in a freshwater lake. A diver who has weighted for the brackish Grevelingen lake and then moves to a freshwater quarry must recheck their lead.
| Medium | Approximate density | Consequence for the diver |
|---|---|---|
| Air (sea level) | ~1.2 kg/m³ | Reference |
| Fresh water | ~1000 kg/m³ | Less lead needed |
| Seawater | ~1025 kg/m³ | More lead needed, better buoyancy |
Swimming in the sea feels "more buoyant" than in an inland lake. That is not imagination: the salt makes the water literally denser, so it pushes you up harder. In the extremely salty Dead Sea you can even float while reading.
Hydrostatic pressure
The pressure water exerts on you comes from the weight of all the water above you. Because water is so dense, that adds up quickly: every 10 metres of seawater adds about 1 bar, on top of the ~1 bar of atmospheric pressure at the surface. The crucial, counter-intuitive insight: pressure changes relatively fastest just below the surface. From 0 to 10 m the pressure doubles (+100%); from 30 to 40 m it rises by only a quarter (+25%). That is why air volumes (in your ears, lungs and mask) expand and shrink most strongly in those top ten metres. This is the physical reason behind the equalising and ascent rules.
Buoyancy and Archimedes' principle
Archimedes' principle: a submerged object experiences an upward force equal to the weight of the water it displaces. Compare that weight to your own: displace more water-weight than you weigh and you are positive (rise); displace exactly your weight and you are neutral (hover); displace less and you are negative (sink).
As a diver you manipulate both sides of this balance. You increase your weight with lead; you increase or decrease your displaced volume with your BCD and your lungs. Your wetsuit plays a part too: neoprene is full of gas bubbles and is therefore positively buoyant, but those bubbles are compressed at depth (Boyle's Law), so your suit provides less buoyancy the deeper you go. This is why you become heavier deeper and lighter towards the end of the dive with an empty, lighter tank.
Steel versus aluminium tank. A steel tank is more negative than an aluminium one, even when empty; an aluminium tank actually becomes slightly positive at the end of the dive. That is why you generally need a little more lead with aluminium to hold a safety stop at 5 m on a near-empty tank.
Heat: why you cool down underwater
Water conducts heat about 25 times faster than air. Your body therefore loses heat to water far faster than to air of the same temperature. Air at 22°C feels pleasant; water at 22°C feels cold after a while, and unpleasant after a long dive. There are three routes of heat loss: conduction (direct contact with the cold water — by far the largest factor, which is why you insulate with a wetsuit or drysuit); respiration (you inhale cold, compressed air, warm it, and exhale that heat — this loss grows at depth); and convection (current and movement carry warmed water away and replace it with cold water).
Stand for a minute in 15°C air: fine. Step into a 15°C bath: almost unbearable. Same temperature, completely different sensation, purely because water removes heat so much faster.
Scenario. In Tofo (Mozambique) you make a long 55-minute manta dive in 24°C water. On paper "warm tropical water", but the duration, depth and current leave you shivering at the surface. The physics explains exactly why: prolonged conduction plus convection from the current. A 3 mm shorty was the wrong choice here; a 5 mm full suit would have been better.
Light underwater: refraction and absorption
Two separate phenomena determine what you see. Refraction: light bends as it passes from water into the air in your mask. The result: objects look about 33% bigger and 25% closer than they really are. A 30 cm fish looks 40 cm; a wall 4 m away looks 3 m away. Experienced divers and underwater photographers learn to "subtract" this. Colour absorption: water absorbs light colour by colour, starting with the long wavelengths. Red disappears first (often within ~5 m), then orange and yellow, then green. Blue and violet penetrate deepest, which is why everything looks blue-green at depth and you only see true colours again with a dive light or flash.
Pro tip (photography). A bleeding fish or a red dive buoy looks grey/black at 20 m, not red. If you want true colours in photos, you must bring light (flash/video light) or get very close so the light passes through less water.
Sound underwater
Sound travels through water at about 1500 m/s, over four times faster than the ~340 m/s in air. This has two practical consequences. First, you hear things well — a tap on your tank, a boat engine, the clicking of shrimp; underwater, occasionally communicating (tapping) carries better than shouting. Second, you judge direction poorly. Your brain normally works out direction from the tiny time difference with which a sound reaches your two ears. Underwater, sound is so fast that this difference almost vanishes, so an approaching boat seems to come from "everywhere". This is a real safety point: do not rely on your hearing for boat traffic, but on your dive plan, your buoy and your ascent procedure.
Bridge to the gas laws
Everything above — pressure, density, the compression of your suit and lungs — comes together in the behaviour of gases under pressure. Three laws govern that: Boyle (pressure and volume: why cavities shrink and expand, the core of barotrauma), Dalton (partial pressures: why oxygen becomes toxic at depth and why nitrox exists), and Henry (dissolving of gas: why nitrogen dissolves in your tissues and decompression is needed). See this topic as the physical foundation; the pressure laws add the calculation rules.
Real-world examples
The Netherlands (Grevelingen / Oosterschelde). Variable salinity means variable density and therefore variable lead. Cold water (sometimes below 10°C) makes heat loss the dominant theme: drysuit, good gloves, short dive times.
Egypt (Red Sea). Very salty, so relatively much lead, but crystal-clear water so light penetrates deep and colours stay visible longer. Still, on a wreck at 30 m everything is blue; bring a light to restore colour.
Indonesia (Lembeh, Bali). Often plankton-rich water: more suspended particles scatter light, lowering visibility and dimming colour even faster. Photographers work very close with strong light.
Maldives. Warm, clear, salty. Ideal to "feel" refraction: mantas look enormous and close, which they partly are, but the 33% effect amplifies the impression.
Mozambique (Tofo and Vilanculos). At Tofo the water can suddenly be colder than expected for the tropics due to oceanic upwelling — exactly the scenario above: do not underestimate heat loss. At Vilanculos and the Bazaruto sandbanks the water is often shallower and warmer, with highly variable visibility from sand and tide; here you notice how suspended particles scatter light and dim colour.
Common mistakes
| Mistake | What goes wrong | How to avoid it |
|---|---|---|
| Same lead in fresh and salt water | Too light (salt→fresh) or too heavy (fresh→salt) | Do a new weight check for each water type |
| Underestimating heat loss in "warm" water | Hypothermia on long/deep dives | Choose insulation for duration and depth, not surface temperature |
| Misjudging distance/size | Missing walls, poor photo composition | Consciously reckon with ~33% bigger / ~25% closer |
| Expecting colour at depth | Disappointing photos, missed detail | Bring light or get closer |
| Judging boat direction by ear | Dangerous ascent in a shipping lane | Trust plan, buoy and procedure, not your ears |
| Forgetting neoprene buoyancy loss at depth | Too light deep, too heavy shallow | Understand that your suit compresses; trim actively |
Pro tips
- Factor in refraction by default. Tell yourself: "what I see is about a third smaller and a quarter further than it looks."
- Choose insulation for the hardest factor of the dive (duration, depth, current), not for the surface temperature.
- Always carry a small light, even by day. Under overhangs, in wrecks and deeper than ~15 m it brings back colour and detail.
- Use taps instead of shouting to alert your buddy; sound carries excellently, but agree beforehand what a signal means.
- Recalculate your lead whenever you change water type, suit or tank type (steel/aluminium).
Frequently asked questions
Why do you need more lead in salt water than in fresh water? Seawater is denser (~1025 vs. ~1000 kg/m³), so it displaces a greater weight and provides more upward force. To stay neutral you must offset that with extra lead.
A fish looks 40 cm through my mask. How long is it really, and why? About 30 cm. Refraction at the water/air boundary magnifies images by roughly 33% and makes them appear ~25% closer.
Why do you cool down faster in 24°C water than in 24°C air? Water conducts heat about 25 times faster than air, so your body loses heat far faster through conduction (and convection when there is current).
Which colour disappears first with depth, and which lasts longest? Red disappears first (often around 5 m); blue/violet penetrates deepest.
Why can't you tell the direction of an approaching boat underwater? Sound travels ~4x faster in water (~1500 m/s), so the time difference between your two ears almost vanishes and your brain cannot work out the direction.
Summary
All dive physics comes back to one property: water is about 800 times denser than air. From that follows the fast pressure rise (~1 bar per 10 m, strongest just below the surface), the buoyant force of Archimedes (which you play with using lead, BCD and lungs), and the fast heat loss (water conducts heat ~25x faster than air, so even "warm" tropical water cools you on long dives). Salt water is denser than fresh and needs more lead. Light bends in your mask, so objects look ~33% bigger and ~25% closer, and water absorbs colour from red to blue, so you bring light at depth to restore it. Sound travels ~4x faster than in air: you hear well but judge direction poorly, a real safety point around boat traffic. All these phenomena come together in the behaviour of gases under pressure, which the pressure-laws topic develops with Boyle, Dalton and Henry.
The 10 things every diver should know by heart
- Water is ~800x denser than air; from that follow pressure, buoyancy, heat loss and sound.
- Pressure rises ~1 bar per 10 m and changes fastest just below the surface.
- Seawater is denser than fresh water and needs more lead.
- Archimedes: upward force = weight of displaced water; positive/neutral/negative follows from the balance with your weight.
- Neoprene compresses at depth; your suit provides less buoyancy deeper.
- Water removes heat ~25x faster than air; even "warm" water cools you on long dives.
- Light bends: objects look ~33% bigger and ~25% closer.
- Colours fade from red to blue; bring light for colour and detail.
- Sound travels ~4x faster in water: clearly audible, but direction is unreliable (boat safety).
- This physics leads into the gas laws (Boyle, Dalton, Henry).