Looks like a giant natural protein skimmer. Used in smaller form to clean saltwater aquariums.
Exactly. A protein skimmer (also called a foam fractioner) was designed to mimic the "motion of the ocean" which creates the froth that carries the impurities away. (And that's really, really simplifying it.)
So these happy beachgoers are really frolicking in ocean waste products. Yaaay!