Ok first let's start with the basics, you need: stone blocks (have to be twice the depth of the ocean) Ladders, to go up and down the tunnel Torches, we don't want a skeleton to pull you down the stairs Step 1: find a good place to start building (at the bottom of the ocean) immediately put blocks forming a square with a hole in the center. Step 2: extend the structure to the ocean surface. Step 3: empty the water with a bucket step 4: use the ladders to descend to the bottom of the structure and dig a bit until you reach an area that no longer has water around it. Done, now you have an underwater drilling area, or as I say: hole to mine underwater. The most recommended is to build a base around the entrance so that you do not fall into the water.