Indestructible Super Shrimp
Aquatic animals brethe the water around them and this equalizes their body pressure with that of the surrounding environment. 

This is how you can still find say a piece of pottery or a watch from a shipwreck... They are filled with water and don't experience the pressure. 

Such pressures do affect humans because we breathe air and so we can't adapt to pressures underwater and we need a shell that allows the ship environment to be tolerable for humans beyond a couple of hundred feet or so. 

I think the change of pressure would also affect fish or other animals If they dropped in depth too quickly.