Although some marine mammals are known to drink seawater at least on occasion, it is not well established that they routinely do so. They have other options: sea-dwelling mammals can get water through their food, and they can produce it internally from the metabolic breakdown of food (water is one of the by-products of carbohydrate and fat metabolism).
The salt content of the blood and other body fluids of marine mammals is not very different from that of terrestrial mammals or any other vertebrates: it is about one third as salty as seawater. Because a vertebrate that drinks seawater is imbibing something three times saltier than its blood, it must get rid of the excess salt by producing very salty urine. In the seal and sea lion species, for which measurements exist, the animals' urine contains up to two and a half times more salt than seawater does and seven or eight times more salt than their blood.
Salt and water management in mammalian kidneys is a two-step process. First the blood passes through a microfilter system in a part of the kidney known as the glomerulus. Most of the blood plasma, including water and small molecules like salts, passes through the filter, but the larger molecules, as well as the blood cells, are held back. The filtered plasma then passes through a long tube called the loop of Henle, where the water is reabsorbed. This process concentrates the remaining fluid, which is finally excreted as urine. One popular theory holds that a simple modification of the standard mammalian kidney¿namely, longer loops of Henle¿allows marine mammals to produce a more concentrated urine by reclaiming more of the water. Kidney anatomy in manatees and harbor porpoises seems to support this theory, but it has not been closely studied in most marine mammal species.
Some species of seals and sea lions apparently do drink seawater at least occasionally, as do common dolphins and sea otters, but the practice is very rare in some other species. When given the choice, manatees and some pinnipeds will drink fresh water. (People who live on salt or brackish waterways in Florida sometimes leave a garden hose flowing into the water in order to see the manatees come to drink). Likewise, some seals will eat snow to get fresh water. For most whales and dolphins, however, we simply do not know how they get their water, because it is difficult to observe these animals.