Somebody may want to check my math before they drown their helicopter, but this is what I get;
water = 62.4 lb / cu ft = .036 lb / cu in = 16.33 g / cu in.
and a cigar tube that is 1.5" diameter X 10" long (big cigar) will hold 7.5 cu in. (radius x length)
And you'll need two tubes or the heli will fall over so that's 15 cu in
So 15 cu in X 16.33g = 244.95 g

And the heli is floating!
Somewhere in here the heli is getting goofy looking because the pontoons are 97% submerged. If we want the heli to sit with the pontoons only half submerged, they need to be even larger. That's based on my heli with the xtreme grips, blades, motors, Lipo, and stock flybar and cockpit with a boom at 238 grams.

Maybe we just need to glue a boat to the heli. It wouldn't look any goofier.