Underwater Floater #MakeItFloat

thingiverse

Here is essentially a buoy, except it has a compartment to put quarters in the shaft. There is a hole in the top, going through the air compartment. The upper bit of the floater is designed with displacement in mind, and the fact that the quarters are almost 6 inches under the water surface ensures a low center of balance and seals to fact that the floater does not capsize.