Evolution is a blind, dumb optimizer. You can have a mutation that is over-kill and if it doesn't actively impede you in some way, it just stays. It's not like it goes, "Ok we need to reduce this to the point where it's just beneficial enough etc".

That said, i definitely would not say the Ocean is particularly less diverse or demanding.

Even with our limited understanding, there must be adaptations for Pressure, Salinity, light, Energy, Buoyancy, Underwater Current etc that all vary significantly by depth and location.

And the bottlenose dolphin for instance lives in every ocean of the world except the Arctic and the Antarctic oceans.