Minecraft version: 1.16.5.
Modloader: Forge
Forge version: doesn't really matter
Environment: Singleplayer

Mod name: Bottled Air
Mod version: 1.0

When you use an empty bottle underwater, it replenishes your air and turns into a water bottle. But you can then drink the water from the bottle and have your empty bottle back. This way, a single bottle is enough to stay underwater forever.

I see two solutions to this problem.

1. Make drinking water from bottles impossible underwater
2. Make it, so that when you drink water underwater, it takes away the same amount of air you got from the empty bottle

I think, the first option is better, as it is more intuitive, but the second one is still a valid choice.

This has been fixed in version 1.6 of bottled air. Thanks for giving the mod a try and opening the issue!

https://www.curseforge.com/minecraft/mc-mods/bottled-air
https://www.curseforge.com/minecraft/mc-mods/bottled-air-fabric

I've added the disableWaterConsumptionUnderwater config option, enabled by default, which prevents players from drinking water bottles while being under water.

This exploit also exists on Fabric. Just be careful to not hold a fire-based item in your offhand, lest the air bottle be melted!
Another method would be to simply reduce the amount of air that the bottle gives you down to two "bubbles". Although this doesn't remove the exploit by a long shot, it does force the player to hold the bottle almost constantly.