Re: Starcraft and floatation foam
if you use expando, consider lining the hull first with saran wrap so you can get it out later if you need to. you'll lose any structural rigidity foam might give you but i suspect would still get the sound deadening since the stuff is still in near 100% contact with the hull.
given that the expando these days is closed cell urethane, i'm not sure it is the end of the world to put it in permanently IF you're sure you'll never need to work on your hull or fix leaky rivets again.