The best way to camouflage stretch marks is to use body makeup, which is a little different than makeup you buy for your face. Body makeup is formulated to work like a foundation, but it has additional ingredients that make it resistant to being rubbed or sweated off. It's typically used by makeup pros, so it's more likely to be found at a department store makeup counter or a specialty makeup store than a drug store. Just as with foundation and concealer for your face, be sure to choose a shade that most closely matches your skin tone.
Before you apply the makeup, cleanse your skin well to get rid of excess oils. Using a cleanser with a mild abrasive, like a salt or sugar scrub, will help remove dry skin cells and expose the freshest skin. Apply the body makeup with a small, soft bristle brush, and then use a sponge to blend it into your skin until you can't see any lines. Try to stay close to the stretch marks -- you don't need to cover the entire area of the body where the marks are located. Set it with a small amount of translucent finishing powder to help keep the makeup in place. Then put on your bathing suit and hit the sand. Just be ready to reapply if the sea or pool takes some of the makeup off.