Thank you for your insights. We will only be repairing the port side of the boat - the other side is dry and solid. Would it be bad to replace one side of the boat with a different core material than the other half?
Also, our cabin interior is currently undergoing a rehaul (read, it's a complete mess), and we were planning on redoing the interior ceiling anyway (peeling paint, stains, sharp, poorly laid fiberglass surface, very yucky), so it's much more appealing doing the work from the inside, especially since one of the few unblemished parts of our boat is the deck. Also, there's the weather issue. In your opinion, is it completely impossible to do a halfway decent job from the under side? I say "decent" - I don't expect perfection.