Depends on if it's a freshwater fish or a saltwater fish.
Freshwater fish are always absorbing water from their surroundings, because their cells and blood are saltier than water. So they don't really need to actively drink it.
Saltwater bony fish (sharks are different) are always drinking water, because their bodies are less salty than the water around them. so water constantly leaves their bodies and they have to drink more to replace it (plus excrete a lot of extra salt).