Fish do drink water! But the methods by which they consume it depends on what type of water they live in!
Water passes into the fish’s body by a process called osmosis. Osmosis is the process in which water diffuses from a higher to a lower concentration. For example: if the water is more dilute on the outside of a cell, water will move into the cell until there is the same concentration of water on either side of the cell’s membrane. The fish acts the same way, either absorbing or losing water depending on its surroundings.
Fish that live in fresh water have a higher concentration of salt in their bodies than the surrounding water, therefore water is constantly moving into their bodies by osmosis. This means, freshwater fish do not actively drink water, but absorb water through their skin and gills. Since fish cannot allow their salt content to be diminished, their kidneys work hard to expel excess water in the form of urine.
For marine or salt water fish, the problem is reversed. Their bodies contain a relatively lower concentration of salt than the ocean water, meaning osmosis causes the fish to constantly lose water. Therefore saltwater fish do actively drink sea water.