Fish, like nearly all living organisms, need to consume water in order to survive. However, fish do not feel thirsty in the same way that humans do. Instead, their bodies have an efficient process that allows them to maintain hydration.
Freshwater fish consume water through their bodies by a process called osmosis. This is the means by which water diffuses through a surface. It is carried out so that there is an equilibrium between the water inside and the water outside a cell. For example, if there is a higher concentration of water outside the cells, then water diffuses into the cells with lower water concentrations until the amount of water on either side is equal. Fish are constantly either absorbing or losing water through their bodies, depending on the amount of salt they have inside their bodies.
Saltwater fish, on the other hand, live in environments with much higher concentrations of salt. This means that they are constantly losing water to their environment. They typically drink large amounts of water through their mouths in order to maintain an equilibrium of salt concentrations within their bodies. Most saltwater fish have special glands and cells that absorb, process and excrete excess salt.