'A' is a container submerged in water with an expandable top portion 'E'. The overall density is (slightly) higher than the density of water (e.g. 'A' is non-buoyant). The container is filled with air under pressure to such an extend that 'E' will just move upwards against the hydrostatic force as per the water level 'F'
'B' is the same container whereby the top portion 'E' has now expanded. As a result the overall density has decreased to less than the density of water (e.g. the container is buoyant). The container will start to rotate clockwise on te pivot point which is located above the center of mass.
'C' the container has rotated to a horizontal position. Due to the increased hydrostatic force the top portion E will be compressed. As a result the overall density of the container will be (slightly) higher than the density of water.
'D' the container will start to rotate anti-clockwise on the pivot point located to the right of the center of mass resulting in the beginning position 'A'
The container and the top portion 'F' will be locked in position when needed during the process.
Why can't this work?
Let me know if anything is not clear.