It's called the tide. The moon makes the water move in and out in waves. It eats away at the sand... it eats away at everything, given enough time. Thousands of years, don't worry about it doing anything now.

[Once they get a little further out, they can begin to float. Since Rue is smaller, Death only takes her out till she's on her tiptoes, while she tucks her feet under her and floats.]