The full explanation can get a little lengthy, so here's the short version: Organisms have to deal with differences in the concentration of salt (salinity) between their internal body and the surrounding water. The bigger the difference, the bigger the "osmotic gradient". This difference also determines the direction in which most water will flow. Freshwater organisms must have mechanisms to keep excess water out, as the fresh water tends to move more rapidly into their "saltier" body. Marine organisms have just the opposite problem and nature has given them the ability to retain bodily water in a salty environment.
An ocean fish, placed in fresh water would drown.
A fresh water fish placed in seawater would dehydrate.
Some fish can tolerate both environments, like salmon & shad...many more.