I just want to note that chest compression only resuscitation is NOT for drowning or near drowning victims. In those cases, attempting ventilation is still recommended. That said, I work in an ER, and I have seen how difficult it is to deliver effective breaths, even when standing on dry land, behind a dry patient, with a proper mask and training. The idea that someone in the water can rear up over the diver, establish a good mask seal, and deliver any kind of effective ventilation is to me completely absurd. If someone is in the water and not breathing, they're in really serious trouble, because they almost certainly don't have a pulse, either. GET THEM TO A SURFACE WHERE YOU CAN DO CPR.