Why do people get thirsty after swimming in the ocean?

Answer 1

The ocean is very salty and salt dries out your mouth giving you the feeling that you are thirsty.

Actually, the reason is two fold:

(1) As with any exercise, you sweat while you swim, as a way to help cool your body, as exercise has increased the body temperature. It may seem amazing, but a typical competitive swimmer can easily lose several pounds of water weight during a typical day's 2 hour swim practice. This loss of water means you feel thirsty, in the same way that you would after an extended run. Look at most swim practices - swimmers will actually keep bottles of water at the end of the pool, and drink from them periodically.

(2) The human skin is not a waterproof barrier. The ocean, which contains a high concentration of salt, is out-of-balance with the water inside of the human body. As two adjacent volumes of a liquid with difference concentrations of a solute will try to balance themselves out, the low-salt water inside of the human body attempts to balance itself with the high-salt ocean water. Unfortunately, salt cannot be absorbed through the human skin, which means that the water inside the human body attempts to "balance" out the ocean water by diluting it (which, given the volume difference, is impossible). In effect, the salty ocean water "sucks" out water from the inside of the human body, as the two volumes attempt (futilely) to balance the salt concentration. This processes is slow, but not that slow. Thus, it is easy to become dehydrated while swimming in salt water, as the ocean literally sucks out the water from your body. A human being will die of dehydration while stuck swimming in the ocean far faster than they would by being in the middle of the hottest desert on earth.

Exacerbating (2), most people in the ocean manage to swallow a non-trivial amount (a pint or more) of ocean water if they spent any amount of time in anything other than completely calm water. The extremely high salinity of said water accelerates the dehydration process, as it sucks out water from the body into the digestive tract (to be eventually excreted in a couple of hours).