
Swim back up for air, then drop back down through the water to place a single block perpendicular to your column at the bottom of your column directly under your original hole.

This slab method also has the advantages of preventing mobs from spawning in dimly lit areas. If you craft cobblestone slabs, you can double the surface area you can create with the same amount of raw material.

Once you have a supply of cobblestone, extend your platform to the bottom of the island and collect the dirt, taking care not to mess up the cobble generator. Continue to generate and harvest cobblestone.
