An inverted dome would work. If you put the one way vent at the lowest level that you want air in the dome it will only release the air until water hit it. With it's exit high that the water line it would not expel the water but would expel the air as it reached that point. However you may end up with a sort of air lift and slowly push water through the tube with the air. If that did happen, you could simply put the outlet in a place where we'd want to return excess water that came with it.
Just on quick thinking, if you made a nice acrylic box that sat over whatever free standing platform you wated to use, you could feesibly remove it to clean, then recharge with an air line and one way valves. Seems like a very interestng concept. I may try it but I would know know what amphibious critters to use seeing as I also want fish and know jack squat about amphibians. If people have suggestions, it seems like a very interesting idea to try. I don't see the purpose unless you want more water for fish so please give me an idea as to why we'd do it and we'll go from there.
You would also have to anchor the "dome" otherwise it would simply rise and release the air or flip releasing the air and sink. So the structure would also have to anchor the dome. The structure and dome would have to be removable, so the structure would have to have some weight, like a cinder block or something to keep from floating.
Either way I suppose there would have to be a reason for it.. the submerged for deeper water. It could be done though.