Waterproof concealer ?

i need a waterproof concealer that i can depend on . i need this to cover up some stretch marks at the pool on a school outing . im really self conscious about them , i am using burts bees shea butter & BIO oil but i dont think they will have faded completly by the time its the day to go . soo please i need a concealer that is WATERPROOF and that i can RELY ON 99 % !!!!