Maaaats' answer is excellent, but I thought I'd add one bit for non divers.
When you learn to dive, the most fundamental fact you learn is that the every 10.5m you descend under water adds 1 atmosphere (bar) of pressure. It's customary to round that down to 10m to make the calculations both easier and more conservative. But the important realization from this calculation is that pressure changes more rapidly near the surface. Descending those first 10m doubles the pressure. From 10m to 20m only adds 50%. From 20m to 30m adds even less, 33%.
And it's that percentage change that is most noticeable underwater. You have to equalize the pressures in your ears and sinuses most in the first 10m under water. And since pressure effects your body's ability to deliver oxygen to the brain, that rapid decrease in pressure near the surface when surfacing is where someone low on oxygen will pass out.
This also explains another part of maaaats' answer. Since a freediver is wearing a wetsuit that compresses with pressure (and the lungs also compresses with pressure), their buoyancy will depend on depth. Being neutrally buoyant at 10m deep is a common practice for all but the most experienced freedivers. This means for those first 10m, you actually have to kick your fins to go down or else you'll just float back up to the surface. But beyond 10m, you can just let gravity take you down. And it works the same way coming up. You actually have to kick to get back to 10m, but once you pass that point, the buoyant force starts to help you get to the surface. Experienced freedivers might actually exert themselves less in those last 10m since they've got that assist from buoyancy.
> it's that percentage change that is most noticeable underwater
Ahh! I figured I was missing something, because the pressure change is linear, but yeah... the percentage change is different. Interesting, and thanks for explaining.
When I freedive, I'm often weighted so that I'm neutral buoyant at -10m. So the last 10 meters I will start to float upwards, and thus go faster.
But that's not the issue here. It's the pressure difference when ascending.
> [1] Ascent-induced hypoxia is caused by a drop in ppO2 as ambient pressure is reduced on ascent. The ppO2 at depth, under pressure, may be sufficient to maintain consciousness but only at that depth and not at the reduced pressures in the shallower waters above or at the surface
Because people swim faster when they're close to the top?