Only 12 out of 4000 does make dry drowning sound pretty rare, but as I heard it in the interview, that wasn't quite what he said about all of those 12 having had a blow to the head. Nancy Grace did seem to gloss over this part completely, but he did say that those 12 all either had some kind of head trauma OR inebriation. I don't know why she ignored that last word or why he let it slide. But in any case, it's a moot point really, because as the poster above me here noted, Riley DID have a "blow to the head" shortly before his death (as seen in a released video), and it's known that he was "inebriated."
On a side note, this was the first time I've watched Nancy Grace! I knew her voice, after hearing it in ads for her show, but never watched it. I'm afraid I personally don't like her style, too dramatic for me. Probably effective sometimes though, I bet. I can see her dragging a confession out of someone by just verbally beating them into submission.