I can't be sure if this is merely lag (which I doubt, as this issue happens for extended periods of time, even when the game is running perfectly fine), but as I've been playing this snapshot, I've noticed most of the time I'm swimming in open waters, I regain bubbles of air when swimming in the upper-most layer of water (the one exposed to air). This has never happened to me before, and having been playing on snapshots for the past few months, I think this issue is with this snapshot specifically.

I tested with this bug some more – it happens in every world I enter, but it's not always bugged; there are some few times where the water functions normally. Still, the majority of the time, open water is bugged.