Print with tree support on bed only (40°) and minimal 3 outher walls*, NO infill (0%)!

Standard size is outside diameter 20mm.

* Scale up for both visibility and buoyancy, see the graph below (based on 0.4 nozzle size):

Neatly trim the remains of support on the sphere with a hobby knife. 

If you plan to sand, use at least 5 outer walls to avoid sanding through the layers / or affecting the impermeability....