Aft Decks....
As you can see, I rearranged the things on the back decks from when I bought the boat. Originally, it had 2 tanks and 1 battery. Now I have 2 batteries (one dedicated deep cycle for the helm/electronics/etc) and one that is dedicated to starting the motor, but I have the option of tying it into the helm with a switch should the other battery die and I need to call for help. I have installed a selector that will allow me to switch between them or have them both tied in at the same time. I know this is probably overkill, but I can see my son somehow taking down the battery and we can't start it to get home. (I have that kind of luck).

The open spot on the right side of the starboard deck will hold a 5 gallon bucket of fresh water for the small sink mentioned earlier.
As a backup plan, I can bring the batteries in the fence in one of the seat bases. There is a 3 inch hole in the fence that lines up with the base and I can vent the base out the back with no issues if needed. That would allow me to put in the second tank again if I wanted.