Yes, it came to my mind but then I found out that shoes with oxygen don't sound good. It's the best solution as for now though, maybe not counting bubbler (but that's not a pickup, so it can't be easily made a one-time thing).

Additionally I think that breathing underwater should be possible when a player has a water shield.

Edit: Extra Time pickup (from what I know) does nothing as well, and sounds a little better than the oxygen shoes for me. It uses the same sprite as the Stopwatch does though, what can be problematic.