I just bought a Hobie 18 for parts and the previous owner had filled the hulls with foam. He did a few things wrong. First, he COMPLETELY filled the hulls with foam. bad news. Second, he didn't allow for drainage by putting some kind of tubing along the bottom. Last, he left the port hole covers off and the foam soaked up all sorts of water. I couldn't pick up ONE of the hulls! I ended up taking a sawzall to it and could barely lift the center 1/3 section around the dagger well!

I wouldn't suggest the Great Stuff foam because it expands and could crack your hull. Also, I'm not sure if it would soak up water or not. You could be inviting a more serious problem by haveing a lot of moisture inside the hulls.

I don't think a Prindle will sink--it may completely fill up with water and turn over, but it shouldn't sink. The Hobies, on the other hand, do have a big chunk of styrofoam in the hulls for flotation but it is loose in the hulls. There isn't a whole lot of styrofoam and it probably only provides marginal floatation--enough for an added measure of security, but probably not necessary.