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. Simply drag and drop it into your mods folder and you're done.
This also adds a gamerule for toggling between 1.12 and 1.13+ air regen 'toggleOldAirReplenish' which is set to true by default.
