airplanes "fly" by making the air they're going through go through a longer route on their top side, /--\, than their bottom side, ___, but in the same amount of time, thereby creating lower pressure above them: the pressure difference raises them. Problem is you need to keep moving.

But imagine for a moment if instead of the airplane we turned to ... the dirigible.

Imagine if you could change the average density of the WHOLE body to be less than the surrounding "air" (water).

You would have to somehow find something that weighed less than water.

In a normal dirigible, where you have to find something lighter than the surrounding air, you fill it with helium/hydrogen/or hot air (which is less dense). But for an "underwater" dirigible?

Well since AIR is ligher than water, imagine if you had hollow sections filled with AIR!

You could have balloons of air, let's call these "ballasts". If you wanted to go lower in the water, you would fill these "ballasts", with seawater from "outside", and when you need to "ascend" again you could drive the water back out (filling the "ballast" from tanks of compressed air).

It might just be crazy enough to work!