That makes sense.
For some reason, when I was reading "Half moon" I was envisioning the bell of a mushroom, Must have been thinking crescent moon.
TW,
Have you give any thought to using a perforated, plastic shell (think a whiffle ball the size of a turn buoy).
It would look, kind of like a mushroom [go figure]. It would be low mass, would not require much flotation to
keep it on the surface or return it once being submerged, and the holes should allow it to move through the
water without causing too much drag.