Minecraft wiki entry for Acquiring_a_conduit, associated text: 1 Heart of the Sea
1.1 Step 1: Underwater breathing
1.2 Step 2: Treasure maps
1.3 Step 3: Following the map
2 Nautilus shells
2.1 Method 1: Fishing
2.2 Method 2: Drowned
2.3 Method 3: Trading
3 Prismarine
3.1 Step 1: Craft a conduit
3.2 Step 2: Get temporary underwater breathing
3.3 Step 3: Obtain prismarine