Minecraft wiki entry for Drowned_farming, associated text: The most efficient way to start the tower over the ocean is to row a boat out to a deep spot with kelp growing near the water surface, allowing you to build from the top of the kelp without needing to build your tower all the way from the ocean floor, and without needing to swim. You can also put a lily pad on the water to start building from. Or, with a bit more materials and time, you can build a sea-level path from your land base out to your build location, and start building a tower out of any material, as high as you need. If you forget some materials, simply jump into the ocean and replenish. You can build a ladder up one side as you build the tower, or if you have enough material and time on your hands, your tower can be a staircase.