This video is one I recognize from the series of pool rescues that was on HN a few days back. Having watched a number of videos from that wave pool, I can confirm that "kid flips tube after climbing on it and is dumped face down" is probably the #1 thing that leads to a rescue.

That said, if you're watching for them, I just sort of defocus to watch the whole pool at once and look for the particular splashes of the drowning person's "swim" that become distinctive after you've seen it enough times.