Can fish drink salt water in the ocean?

Marine bony fishes such as tuna, flounder, and halibut drink seawater almost constantly to replace water lost by osmosis and through their gills. It is estimated that they drink an amount equal to 1 percent of their body weight each hour, an amount comparable to a human drinking 1.5 pints or nearly three cups (700 milliliters) of water every hour around the clock. The gills eliminate most of the excess salts obtained by drinking large quantities of seawater. The fishes excrete small quantities of urine that is isotonic to their body fluids. By contrast, cartilaginous fishes (e.g., sharks and rays) do not need to drink water to maintain the balance of water (osmotic balance) in their bodies. They reabsorb the waste product urea, creating and maintaining a blood urea concentration that is 100 times higher than that of mammals. Their kidneys and gills thus do not have to remove large quantities of salts from their bodies.