Minecraft wiki entry for Underwater_home, associated text: The "lily pad" method creates a 3x3 column to the bottom by dropping sand off the four edges of a lily pad from a boat on the surface. Alternatively, you can place a block onto the lily pad to make it easier to place the sand. Once you have an enclosed 1 block column of water you fill the central block with more sand, then work your way down from the top digging out the central column of sand and replace at least one of the sides with a non-falling block, like dirt or wool, and place ladder blocks on the non-falling block. When you reach the bottom you can dig out through the solid wall (it will not collapse) and place a door to keep the water out. Or you can dig down further into the seabed and make your base entirely below water.