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
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Brazmann
Owner
Technical information
License
MIT
Client side
unsupported
Server side
required
Project ID