It would be far easier just to seal the mast. If nothing else, foam in the mast will add weight aloft, which is never a good thing. As flotation results from weighing less than the water displaced, it would seem that adding foam to the inside of the mast could make things worse, as foam weighs more than air - you'll be increasing weight for the same amount of displacement. The only benefit would be that it could keep water out - but only if it's a closed-cell type that is not going to break down in the constant flexing of the mast. If it breaks down and your mast is not sealed, it will absorb the water.



Adding a float to the outside of the mast increases displacement, which is why they would work better. But you're still adding weight and now drag aloft.



Seal the mast. If you want more flotation, add a float of some kind, but I wouldn't mess with filling the mast.