Instant Air Replenish

As of Minecraft version 1.13 and beyond, player oxygen regenerates over time out of water as opposed to happening instantaneously.

This mod restores that pre-1.13 functionality with a gamerule 'toggleOldAirReplenish'. It's set to true by default, and you can use the following command as an op to change it:

/gamerule toggleOldAirReplenish true/false