Re: can i use the great stuff expanding foam from home depot?
Its not the best choice if it will come in contact with water. I would use it above the deck, like foaming the underside of gunnels and places like that where pouring foam is imposable. Plus there is only a square foot or so of foam in one of those cans, so its pretty expensive stuff.
If you want to foam below deck, I strongly suggest using the marine flotation foam. Second choice is the rigid pink foam boards cut to fit the spaces. If you want to glue the foam boards together to make larger blocks to carve, use Gorilla glue.