This modpack makes use of a custom datapack and resourcepack that only allows for the player to breathe underwater (unless you craft a "Poseidon's Blessing" -- a crown that allows the wearer to breathe above water).
Originally from lyndsayrae's hardcore videos:
Recipe for Poseidon's Blessing:

Put the knowledge book from this recipe into your inventory and you will automatically receive Poseidon's Blessing.
Main gameplay mods:
- Mermaid Tail
- Ice and Fire
- Mystic's Biomes
- Wyrmroost
- Upgraded Netherite
- Alex's Mobs
- Better Animals Plus
- BetterNether
- Biomes O' Plenty
- Wings Horns & Hooves
Aesthetics / atmosphere / convenience:
- Dynamic Surroundings
- Sound Filters
- Falling Leaves
- Inventory HUD+
- Blur
- BetterF3
- Perspective Mod Redux
- Tree Harvester
- CleanCut
- Cosmetic Armor Reworked
- Just Enough Items
- Perspective Mod Redux
- Skin Layers 3D
- Sound Filters
- Toast Control
- Xaero's Minimap

