Pond

Mateo realises that he's supposed to have drowned, and that playing his hand too soon will alert the siren that he has survived. So for now, he plays dead, lying still and not moving, waiting for the siren to approach. As noted, he doesn't need to breathe, and so he doesn't, and his eyes are closed. He'll listen for her approach; his hearing is quite good. Much better than a human's, in fact.