Most surface vessels purposely engineer the center of gravity to be slightly above the center of buoyancy for maneuverability. Surface vessels that don't have that property are termed bottom heavy and are difficult to turn. The stability of a surface vessel is usually measured by its metacentric height and determined by the inclining experiment. The reason the ship is stable is that it picks up a restoring moment as the ship rolls one way or another and the water plane increases and so you'll have a roll frequency develop. Too high and the crew gets easily seasick.
Submarines are a different matter. There is no water plane when the sub is submerged and the metacenter merges with the center of buoyancy. Surfacing is a critical operation, and if not done carefully it can cause the submarine to roll over during the transition of metacenter.
Your camera problem is the submarine problem and you don't need to concern yourself with metacenter since it merges with the point of buoyancy. Your righting arm is strictly the difference between the c.g. and c.b. assuming your camera body is rigid.