Do tampons work in water for someone with a HEAVY flow?

So this fourth of july, my family is coming over, as usual. We ALWAYS go swimming together and this day is just about as important as Christmas to me. My period is coming on that day, and it's usually very very heavy. I've never used a tampon before and im scared because i just want to make sure that it WONT leak in the pool or it wont make me embarrassed in any way(like, i dont want it to slip or come out or something, but im worried about leaking the most). PLEAAASE help and thanks for any answers((: