[countshock]

Thanks guys, we'll see what I find.
I didn't know if those chunks of foam were needed. I removed all the foam around the tank itself, but there were still those big squares fore and aft, under the deck.

I'll definitely put some strips down and drill some limber holes.
It may be rotten, but if feels really solid. Jump up and down on it solid.
Thanks for all the suggestions guys.

Keep 'em coming.