Do you know if fish drink water? Some actually do, depending on the quantity of the salt in their bodies compared to the quantity of salt in their environments.
Saltwater fish, for example, have only one third the amount of salt in their bodies that is in their environment. Because of this, they lose water due to a process called osmosis, which causes water to move through the fish’s gills and skins. As the water moves from areas of low to high salt concentration, these fish must gulp water to replenish their supply.
Unlike saltwater fish, fresh water fish do not have to drink water. These fish have a higher level of salt in their bodies than in their environment. Therefore, water is consistently brought into rather than removed from their bodies. Freshwater fish get rid of extra water in their bodies by urinating.
Not all fish are able to control the amount of salt in their bodies, however. For instance, the hagfish—a saltwater species—must stay within a specific range of salinity, or salt level, to survive.
[Source: Science Illustrated]