We can’t drink sea water because of high salt content . But whales, mantees, seals and other sea faring mammals like earless seals, sea cows, sea otters and polar bears stay hydrated.
The short answer is:
A. They get water from their food, and avoid salty food.
B. They may have modification to their kidneys to allow them to excrete more salt.
C. There a lot we don’t know, marine animals are hard to study.
Source and more details
Copy from “Marine Mammals Evolutionary Biology” by Annalisa Berta et al. I worked closely with her and she is one of the foremost experts on evolution and systematics of marine mammals. I will add personal clarifying results in [brackets].
Most marine mammals are hypoosmotic; their body fluids have a lower ionic [salt] content than their surrounding seawater environment, and they are constantly losing some water to the hyperosmotic seawater in which they live.
Marine mammals obtain the water they need from the food they eat: preformed water in their diet [as previously mentioned in this thread] and subsequent metabolically derived water [not previously mentioned]. Most fish and invertebrate prey consists of 60-80% water, and the metabolism of fat, protein, and carbohydrates provide metabolic water during the digestion of food. It has been shown experimentally that seals [also referred to as phocids] can obtain all the water they need from the food they eat. If seawater is given to seals, the stomach becomes upset and the excess salts have to be eliminated using body water.
Despite this, seals occupying warm climates have been observed drinking seawater (King, 1983), a practice called mariposia. It has been suggested that intermittently consume small amounts of seawater at intervals that would not be enough to cause digestive problems, but would be sufficient for facilitating nitrogen excretion [peeing]. Mariposia is especially common among adult male otariids [sea lions and fur seals] (Riedman, 1990).
Why do pinnipeds [seals, sea lions, fur seals, and walruses] drink seawater? Gentry (1981) noted that most of the otariids observed ingesting seawater live in warmer climate and lose water by urination, panting, and sweating. He suggested such water loss , along with prolonged fasting by territorial males, may be severe enough to promote the drinking of seawater. The behavior may play a role in nitrogen excretion by supplementing water produced oxidatively from metabolized fat reserves. Mariposia has also been reported for Atlantic bottlenose dolphins, common dolphins, and harbor porpoises. I’m getting tired of typing, but theres about one more page to go. I’ll type it up if anyone wants to read more
- Eat high water content food
- Metabolically produce water from digestion and fat reserves
- Kidneys stronger than desert animals
It’s actually not established that sea mammals drink large amounts of salt water. So they get it from consuming food and peeing out far larger amounts of salt in their urine than humans do.
A theory is that their longer henle’s loop allows better processing of salt water via their kidneys as well.
Rain water will remain on top of salt water estuaries until winds mix them together. Manatees are regularly observed drinking the rain water at the surface. They also love drinking water from hoses.(please don’t give them water, it encourages them to hang around boats, resulting in prop injuries) Manatees have pretty efficient kidneys, but no where close to the efficiency of dolphin and whale kidneys. Manatees seek freshwater sources daily.