ORIGINAL: SV27dayton
I may just use the stuff that was in my SV's box. I think that's the same stuff they put in the bow of it. I replaced it with a thermo sealed bag of air (can't get lower density than that) but for this boat given the mutiple bulkheads I'm going with foam.
I've been trying to find good foam as well, I did find some pretty good looking pipe insulation foam(home depot), it's basically the same as a pool noodle, but these pipes have a smaller OD, are charcoal gray instread of neon and really cheap. They're also really light, I'm pretty sure that's what I'll use. Think it was $3 for about 9ft of the stuff.
I'd stay away from any spray on foam. It might not crack the hull, but it could deform the hull and I dont think it will be very light.