Freshwater fish do not actively drink water, but absorb the water through their skin and gills. On the other hand, saltwater fish do actively drink sea water. Their gills process the water and take out the salt.
The salmon is an interesting example of a fish that lives in both fresh and salt water, and consequently, they have the characteristics of both types of fish. Salmon are born in fresh water, and migrate to the ocean. While living in the ocean, they drink saltwater by opening their mouth; their gills then rid their bodies of the salt and minerals. When the salmon enters freshwater streams to spawn, they stop drinking the water and instead absorb it through osmosis (osmosis means that the water passes through the cells of the fish's skin into its body).