The argument about being able to drink a certain amount of sea water per day may stem from the fact that, normally, people consume (more than) a recommended 3 grams of salt per day with their food. Your body should be able to cope with this amount of salt because it needs it as nutrition. If you are floating around in a life raft, you probably won’t be consuming this level of salt (unless you pack some food along with you, but then you should have packed water, too ). So, maybe your body is able to cope with a certain amount of sea water by simply using the salt as nutrition instead of just getting rid of it through urination.

Of course, that whole spiel was purely half-educated guesswork and could be totally full of crap. Enjoy!