This Minecraft tutorial will show you how to create structures of any size and shape underwater quickly, easily and without too much hassle.
First figure out a shape for your structure
Place a layer of dirt in the shape of your structure. Figure out the height of the structure you want to build. Then place sand or gravel to the height that you want.
Carve out a flat ground or surface
Dig the shape of the base of your structure. You may also place blocks to make a flat surface. Night vision potions work well if you cannot see underwater.
Remove the dirt
Now remove the dirt so that all the sand or gravel falls in the area you shaped under water.
Once that is all done, make an encasing of the structure. As long as all sides are covered after all the sand or gravel is down, it’ll be safe to remove without the interference of water.
If you quick place a torch after digging the sand or gravel away, the stack of sand or gravel will fall on it and be instantly be mined. This is the most efficient way to remove it.