This a a ball that that was designed to support a Cannon Powershot D30 camera in water.

The camera is waterproof, but sinks in fresh water. A real bummer if dropped off a boat.

The camera measures ~ 2-5/8 X 4-1/4 X 1-1/8 and is about 220 grams.

31g of extra buoyancy give enough of a reserve to keep it afloat.