As I believe I mentioned earlier, I intend to put a deck below the upper deck to make it easier to keep water out. I have been doing some progress on this. I made three pieces of polystyrene that I glued together to one long piece that has the same shape as the deck. Then I have been sanding the edges so the lower deck can be placed inside the hull, below the upper deck.
Now I intend to put some magnets below the lower deck and some metal on the downside of the upper deck to keep the upper deck in place. I am still intending to use the 16" gun turrets to make a mechanical fix between the two decks.
Here are some pictures of the lower deck: