I imagine a safe version could be made... that would suck all the direct-competition spontaneity out of it. Like "do it on land", or "only in standing-height water and take turns with someone timing and acting as a spotter."

Yes, although as another commenter pointed out, being underwater triggers a response in our body that lowers our oxygen consumption.

We did spot for each other, even then, but I don't ever remember anyone thinking they almost passed out or anything.