How did I not drown? I used a helmet enchanted with respiration III and a bucket.

How did I do it? I made the form out of sand on the surface above the site by building a cobblestone platform. Then I went below and leveled the sea floor out under the site. Next, I broke up the cobblestone which allowed the sand to fall to the sea floor. Finally, I covered the sand form in glass and went inside and dug it out.

I'd like to do something bigger and deeper in the sea, but that took alot of time.....