In regards to the farming update, we have no idea when it will be released, so I wouldn’t wait for that.
For the actual building, what I did was:
Lay down two layers, starting at ocean level and under it.
------------ (water)
XXXXXX (1st layer)
XXXXXX (2nd layer)
Then, remove the insides of the layers. Leave the “wall” up. You should plan for this wall to be your final material. Continue to add layers as needed.
------------
X - - - - X
X - - - - X
Try to add to the outside walls first, so you can swim up to air. The fill it and dig out. Dirt is the easiest to work with.
If you are building from the ocean floor up, though, I would get a sinking item. Setup a small shelter for yourself, like a 4x4x4 room. You’ll go in there to get air. Then build from the ground up. Exterior first. Once you have the ceiling in place, you can start at the top layer, placing and removing dirt to drain the water.