In my biology class we got the question if saltwater fish need to drink. We need to use the term osmosis, but I cannot think of a valid answer and we were allowed to ask help.
Short answer, yes. The fresh water concentration inside saltwater fish is higher than the surrounding sea water, and the salt concentration inside the fish is lower. This means that fresh water continuously flows out of the fish through their gills via osmosis. To combat this loss of fresh water, fish must constantly be drinking sea water to replenish the lost fresh water. This also requires that they pump out the excess salt in the sea water they drink using their kidneys and specialized chloride cells in their gills.