Bottled Air

Bottled Air is a mod which enables another way to remain under the water for longer. Aside from potions and conduits, you can now gulp air out of an empty glass bottle. Which is done by just right-clicking an empty bottle while swimming, when the air bubble bar is visible. This will by default refill half of the player's air bubbles.

To prevent your inventory from getting completely filled with water bottles during this process, you can hold one of the fire-type items in your offhand. The current fire-type blocks are Campfires, fire charges, lava buckets and magma blocks. When holding one of these blocks, the newly created water bottle will evaporate and you'll get the glass bottle back. By default there is a 50% chance that the glass bottle breaks in this process. Go explore that underwater world!

All available config options:

disableWaterConsumptionUnderwater default true
If enabled, players will be unable to drink water bottles underwater to prevent getting unlimited oxygen.

holdFireTypeItemInOffhandToPreventWaterBottleCreation default true
Whether the creation of water bottles should be prevented (evaporated) when holding a fire type block in the offhand.

chanceGlassBottleBreaksWithFireTypeInOffhand default 0.5, min 0, max 1.0
The chance a glass bottle breaks when the item in the offhand evaporates the water, giving back an empty (air) bottle.

amountOfAirInBottles default 150, min 0, max 1000
The amount of air an empty bottle contains. In vanilla Minecraft, 300 is the maximum air supply.

Jump in and gulp that air out of the empty bottles to swim under water for longer:

The air bubbles bar is filled by half with each glass bottle. There is a 50% chance it breaks during the evaporation process:

Of course you don't have to hold a fire-type block:

You may freely use this mod in any modpack, as long as the download remains hosted within the CurseForge or Modrinth ecosystem.

Serilum.com contains an overview and more information on all mods available.

Comments are disabled as I'm unable to keep track of them across so many separate mod pages.
For issues, ideas, suggestions or anything else there is the Github repo. Thanks!