Cells contain less salt than sea water, and cell membranes are semi-permeable. That leads to osmosis drawing water out of the cells, drying out the fish.
Sea-dwelling creatures have different ways to deal with this. Most fish can "pump" salt from their blood into the water in their gills, in addition to their kidneys filtering salt.
The other strategy is to eat fish that have already done all that work, after all most animals are mostly water of compatible salt content.
(Freshwater fish have the opposite problem, without active salt management they would explode from osmosis drawing water into their cells).
Sea-dwelling creatures have different ways to deal with this. Most fish can "pump" salt from their blood into the water in their gills, in addition to their kidneys filtering salt.
The other strategy is to eat fish that have already done all that work, after all most animals are mostly water of compatible salt content.
(Freshwater fish have the opposite problem, without active salt management they would explode from osmosis drawing water into their cells).