looks like a combo key holder + wax comb since it has that magnet on it and it is box shape right? It could be useful as a key holder even if you don't surf. Just pop it under your car somewhere when you want to jump in the water at the beach.
That piece is obviously designed to defend against sharks. They say you're supposed to punch sharks in the nose if you are under attack under water, until they actually tried punching a shark underwater and realized that their punch amounts to the equivalent of a fierce nose-patting, so they designed these vicious combs.