I thought I read somewhere that he hit his head but I may be wrong. People need to stop judging. He may have been a good swimmer and the parents may have been close by. If someone hits their head they could go under and drown in no time. The parents may have been there and looked away for even a few minutes. It's very possible he hit his head and slipped under and no body noticed for a couple minutes. Thats all it would take. They could have seen him and not realized he was in trouble and thought he was just under the water. People judging need to get out of their glass houses.
My heart goes out to this family. Hope he's ok.