How do people who take ex lax lose weight since the weight will come back if they drink water but they have to drink water to take more ex lax the next day right?

ok im joing the navy so I've really needed to lose quickly at times....i took some xlax and i lost 2 pounds in a day...two days later i was still the same weight...i think the trick is once you have some off, drink alot of water, and eat like a can of tuna for lunch..nothing in it...and a peice of chx breast for dinner nothing on it....protein is really good to help you burn fat...its carbs you wanna stay away from like bread, rice, pretty much anything white..cut out. im curintly doing weight watchers and its been the best thing to me so far I've lost 17 pouds so if i were you i would give that a worl..its way healthyer