Obtain a Brewing Stand
Obtain Blaze Powder to fuel the Brewing Stand
Obtain Water Bottles
Obtain Nether Wart
Brew Nether Wart into the Water Bottles to make Awkward Potions
Obtain a Pufferfish
Brew the Pufferfish into the Awkward Potions to make Potions of Water Breathing
Optional: Add Redstone Dust to extend the duration
Optional: Add Gunpowder to make Splash Potions of Water Breathing
Optional: Add Dragon’s Breath to make Lingering Potions of Water Breathing