Minecraft wiki entry for Achievement_guide, associated text: Drink a potion of water breathing, then jump into water. Or use whirlpool bubble columns from a magma blocks to breath underwater for 2 minutes. Magma blocks can be usually found in underwater trenches, underwater cavern at lava layer, and at underwater ruins. To brew potions of water breathing, you need pufferfish, find the potion in buried treasure, or killing a witch while it is drinking the potion. You can get pufferfish by fishing outside jungle biomes or killing pufferfish which can only spawn in warm ocean.