Here's my thought on how I'd like to do this:
I'm glad you pointed out the size of the lens. With an underwater housing, I'm not convinced you could really get many "split the scene" style shots. The typical chop in the ocean is just too rough, and shots would either tend to be all the way above the water or all the way below.
Rather than use an underwater housing for the transition, I was thinking of making a box about 2'x2' with one side either Lexan or tempered glass. Box goes in the water (weighted down by cinder blocks, rocks, sand, whatever) and the camera is slowly lowered through the transition from air to water with, as you said, an object in the frame that extends from the water to the air. The hope here is that there will be a number of images in the set with a clear split in them, showing above and below.