Basically the same reason it is for humans – it puts a lot of strain on their kidneys. I believe that cetaceans are thought to be able to drink seawater as a la

Basically the same reason it is for humans – it puts a lot of strain on their kidneys. I believe that cetaceans are thought to be able to drink seawater as a last resort, but get most of their water from eating fish. The fish don't have the same salt content as the water that they swim in.