open air box under the ocean

When flying, sailing, or swimming over an ocean you can sometimes see these small black or dark blue boxes under the water. If you investigate you will find that they are small pockets of air in the ocean, under the water, with water on top, sides, and bottom, yet no water flows into it at all. You can fly into it, swim, everything, yet there is no water in that gap. You will most likely find one or multiple if you just fly over an ocean while keeping your eyes out for small dark blue boxes. Or maybe it is just my system.