I would add that since, as wunderhorn1 points out above, fish extract water by passing it over their gills, they can be suffocated (in many cases) by preventing them from moving in relatively still water. Sharks are the most extreme example; their oxygen demands are too high to permit them to circulate water with their gill opening muscles as many fish can. For this reason, unlike most fish, sharks cannot stop moving. If they do, they drown.

This also means they cannot sleep, and in fact do not. This goes a long way to explain their legendary irritability and generally lethal attitude.