Re: What's under the Deck?
You have an older FIsher like I do if the deck is aluminum.

No rot to ever worry about. The company was "absorbed" by tracker marine and then the construction got cheaper with plywood.
I have opened the decking on mine by locating the screws under the carpet. I actually went and got a good size box of matching stainless steel screws and I replace them when I need to. I also put a dab of grease on every screw I install.
I have a tank under the deck of my Fisher but mine is a bass boat. There is a plastic hatch about the tank to access the tank fittings.
You might find when you open things up that Fisher has placed 2 part foam in some spots. This might hamper things.
Post a few photos and we might be able to figure things out. Since the company was taken over there is NOT any manufacturer help available. Tracker is NO HELP at all.