Generally, I expect my oceans in Minecraft to be uniform and filled. This wasn't the case when I went underwater attempting exploration. Large air pockets were suspended underwater, and similar to floating sand bugs, I expect this to be a generation error.

From what I can assume about reproducing this bug, I doubt you'd have to do much more than get unlucky with terrain generation. Although, i was about 500 blocks out in a positive Z direction, which may or may not be a contributor.