Not sure if this would be relevant to your boat design. My boat has a long tank along the keel line amidship. I suspect it would be very difficult to get that compartment water tight and keep it completely dry, taking into consideration even condensation. I am not sure if I would want to even try and seal it in as I think the more it breathes, the better in terms of corrosion control for the Al tank.
There is an under deck bulkhead between the fuel tank compartment and what used to be the motor compartment before the boat was poded. It is low enough that air can circulate under the deck and over the top of the bulkhead to let the Al tank compartment breathe. There are also two very short tubes through the bulkhead right at the bottom. This allows any water that does get into the fuel tank compartment to move aft, where the auto bilge pump will deal with it.
I also have a small round plastic access hatch in my deck fuel tank cover that has its own rubber seal and does not appear to leak. Perhaps you need a new hatch or just the seal. As I recall, those hatches are not very expensive. With a two part hatch, it is one thing to seal in the non-removable part to prevent water leaking under it, and we have used Sikaflex for that on some two part hatches. If it is the seal under the removable part of the hatch that is leaking, you need a new hatch, or at least a new seal ring. That is, if you want to keep it as a functional easy access hatch.
