As far as the foam breaking stuff as it expands, I learned taht lesson a long time ago while filling in a hole behind some drywall... lol swelled out the wall like crazy. I wound up with a big mess. I was planning on epoxying the inside of the boat area, then adding the foam before she was all closed up. I didn't know that the foam would break down. I have been asking folks all over the place what the life expectancy of the Great stuff is, if they ever saw it deteriorate, etc. I suppose the coke bottle idea is a good one, and just pull them out & check for cracks every yr? I could put a few behind the aft bulhead in the transom, too, I guess. Not too hard to figure out how many bottles I would need for floatation, with a known volume! Thanks for all of the comments!