eli5 : Is water near the surface of the ocean less dense than deep water

Question

In other words, does it take more effort for fish to swim in very deep water than in shallow water.

In: 163

Answer 1

The water down below is colder, so that’s one part of it. Water at atmospheric pressure reaches its densest around 4°C.

Water can also be compressed. You may have heard that water is incompressible, but that’s not entirely true. It’s just very very hard to compress, especially compared to air. Anything can be compressed if you push it hard enough, and all of that water on top of it is very heavy, so there’s a lot of force crushing it down.