I scraped this answer, but I thought it was so interesting, I am reblogging it:
Fresh water fish absorb water through their skin and gills, saltwater fish actually do drink water.
In saltwater fish, they have to drink because their body’s concentration of salt is lower than the surrounding water. Therefore, they have to drink huge amounts of water every day to stay hydrated.
In freshwater fish, their salt concentration is higher than that of the surrounding water, and, as osmosis dictates, they absorb water through their highly permeable skin. To keep from bursting, freshwater fish actually have to excrete water, up to 10 times their body weight daily, unlike saltwater fish.