Freshwater fish never drink, because they are always absorbing water through their skins and gills. Their problem is how to get rid of excess water. Saltwater fish, on the other hand, are always losing water through their skins and gills, so they drink more or less constantly.

Osmosis is a movement of liquid through a membrane from a lower to a higher concentration. Diffusion: is the movement of particles from an area of higher concentration to an area of lower of concentration.

In both cases, this is due to osmosis with a permeable membrane (like fish skin), water always moves from the side with a lower salt concentration to the side with a higher salt concentration. Freshwater fish and saltwater fish have about the same concentration of salt in their body tissues, which means that freshwater fish have bodies that are saltier than their surrounding water, and saltwater fish have bodies that are less salty than the surrounding water. If saltwater fish didn't drink the water they would shrivel up ;)