Web research seems to indicate that most fish do drink plenty of water, some continuously. Some fish absorb water through their skin and/or gills, and may excrete water that way, too. Apparently, fish need to maintain a fairly high concentration of salt, so how a fish deals with this need depends on whether it is a saltwater or freshwater fish. Most saltwater fish get enough or too much salt, and so preventing water loss is their concern. Freshwater fish are short on salt, and since water dilutes sodium, they have many mechanisms for excreting water. It is interesting to note that kidneys process water for excretion, so some saltwater fish have dysfunctional kidneys or are missing kidneys to help prevent water loss.