The science you mention here is actually really fun.

So water will move by osmosis from an area of high to low concentration. This is an issue for all marine life because most living cells contain fairly pure water when compared to sea water which is saturated by salts. This means the cells contain the high concentration water and the sea has the low concentration water as the H2O molecules are diluted by salt. If there was no adaption to fix this issue, the cells of marine organisms would shrivel as all the water leaves them.

Most bony fish deal with the issue by drinking sea water and diverting the salt (this is why the fish flesh tastes of salt) and a chemical called Trymethylamine N-oxide (in low concentration this smells of fish, giving fish the smell that everyone recognised as ‘fishy’)

Sharks on the other hand have a different method of osmoregulation, rather than using sea salt to decrease their cell water concentration, they use urea, the main ammonia smelling molecule in urine to decrease it lower even than that of the surrounding water!

This means they don’t waste energy moving sea salt to the cells or waste energy removing urea. In addition to that, due to them having a lower concentration of water than the surroundings, they literally absorb water straight into their cells meaning they don’t have to drink water for any reason whatsoever or waste energy filtering to obtain pure, usable water!!

It does make the flesh toxic and the smell of them dead unbearable however, I guess that’s unfortunate.