Saltwater fish have to drink water almost constantly. Water is constantly leaving their bodies through osmosis because their body fluids are more watery than the sea water in which they live. Because they are constantly losing water, they must also constantly take in water. They take in water mostly by drinking.
Marine mammals don't drink salty water but they eat fish and fish drink salty water, this water being processed through osmosis and diffusion.
Saltwater fish drink water. Since saline water concentration is comparable to that of their body, they need to purchase the water from environment.
Unlike Saltwater fish, freshwater fish get their water through osmotic absorption of the skin and therefore they do not drink water.
If the marine mammals are kept in captivity, they are fed by freshwater. The water is provided from a bottle or through a pipe.