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Fish live in water, but do they drink it?

The answer is yes, but how fish drink water depends on where they live.

Water gets into a fish's body through osmosis, the process in which water diffuses¹ from a higher to a lower concentration. For example, if there is more water outside of a cell than inside, water will try to flow into the cell until there is the same concentration of water on either side of the cell's membrane². The body of a fish acts the same way, either absorbing or losing water depending on its surroundings.

Whether a fish absorbs or loses water is based on the fact that all fish must maintain a certain amount of salt in their bodies to stay healthy. Fish that live in fresh water have a higher concentration of salt in their bodies than the surrounding water. Consequently, water continuously flows into the fish's body to attempt to dilute³ the amount of salt in the fish until it is equal to the amount of salt in the surrounding water. Since fish cannot allow their salt content to be diminished, their kidneys work overtime⁴ to expel excess water in the form of urine.

Ocean fish have the opposite problem. Surrounded by salt water, their bodies contain a relatively⁵ lower concentration of salt than the ocean water. In this case osmosis causes the fish to constantly lose water in order to equalize salt concentration inside and outside the fish. to partially⁶ compensate⁷ for the water loss, ocean fish actually drink water through their mouths. And to get rid of the excess salt they take in by drinking seawater, they excrete some salt through cells in their gills.