This design is a fishing float.
It can be printed in one part without support. I only use a brim to make sure the part doesn't detach from the bed during printing.
3 outer layer with a slight overextrusion (+2%) make for good waterproof parts. 0% infill is doable but i use 10% for mechanical resistance.

I use the full scale model and a 75% scale version for smaller lines.
100% scale floats work with 5 to 9g
75% scale floats work with 1 to 2.5g
(plain water, not salt water)

Nail polish works well to put some color on it.