Question

why do not fishes or other aquatic animals collapse by the presence of water exerted on them?

Answer 1

It is pressure and they do not collapse as their BP is suited to that high pressure

Answer 2

Fish are a class of aquatic vertebrates. The combination of gills, fins and the fact that they live only in the water make fish different from all other animals. And some...not so much. Fish spend all of their lives in the water and are cold-blooded with the exception of Tuna family and the Mackerel shark family.