I filled all the sealed compartments on my MiniCups with packing
peanuts and one large compartment had a crack and took on water and
although I drilled small holes to allow it to drain it always had a
small amount inside. Later, I cut it open and found that although the
peanuts had not soaked up any water, they were all wet and stank like
mildew. Scooping out several cubic feet of mildewy wet packing peanuts
thru a small hole was unpleasant. So, if I did it again, I would seal
the peanuts in a heavy duty trash bag first before stuffing it into the
compartment.
The boatbuilding NG will tell you NOT to use expanding foam as it will
deform things and they are probably right.