I learned about shallow water hypoxia when I first took up SCUBA diving (before most of you were born :)). It occurs during ascent, for two reasons. The most obvious is that you've been holding your breath longer when you're ascending. The other is that when your body is under pressure, the partial pressure of oxygen in your lungs is higher, allowing the hemoglobin to absorb more. As you ascend and the pressure lessens, the partial pressure of oxygen decreases accordingly.

Apparently shallow water hypoxia is now a thing that life guards worry about, despite the fact that it's unlikely in a swimming pool. I swam underwater about fifty feet in my neighborhood pool, which is five feet deep, and was told by the lifeguard never to do that again. Indeed there's a sign that says "don't hold your breath underwater." Maybe you're supposed to breathe underwater?

I blame the American Red Cross, which has a history of dumb things IMO, starting with teaching the crawl stroke to non-swimmers instead of the side stroke or back stroke, both of which require far less coordination, and allow you to keep your face out of the water while swimming.

But don't get me started, or I'll tell you about "drown proofing."