Whales are mammals. So are we. And we need to drink lots of water - the standard recommendation is 6-8 glasses per day, although you can calculate your own needs here. So whales must need to drink water... or do they?
Whales live in the ocean, so they are surrounded by salt water, with no freshwater in sight. As you probably know, us humans can't drink much salt water, because our bodies can't process that much salt. Our relatively simple kidneys would need a lot of fresh water to process the salt, meaning we'd lose more fresh water than we were able to extract from the sea water. This is why we get dehydrated if we drink too much salt water.
Although it's not well known how much they drink, whales are capable of drinking sea water because they have specialized kidneys to process the salt, which is excreted in their urine. Even though they can drink salt water, whales are thought to get the bulk of the water they need from their prey - which includes, fish, krill and copepods. As the whale processes the prey, it extracts water.
In addition, whales need less water than we do. Since they live in a watery environment, they lose less water to their surroundings than a human does (i.e., whales don't sweat like we do, and they lose less water when they exhale). Whales also eat prey that has a salt content similar to the salt content in their blood, which also causes them to need less fresh water.
References and Further Information:
- Garrett, H. Do Whales Drink From the Ocean? ASK Archive. Accessed April 29, 2013.
- Kenney, R. 2001. How Can Sea Mammals Drink Salt Water? Scientific American. Accessed April 29, 2013.
- Perrin, W. F., Wursig, B., and J.G.M. Thewissen, editors. "Osmoregulation," inEncyclopedia of Marine Mammals. Academic Press. pgs. 838-839