I plan to install a water heater in my boat. It never had one, but I can use a storage bin in the engine room. A 6.5gal will fit, but just. A 5 will be easier to install, leave a little storage space. This will use a heat exchanger, not electric. I plan to cruise the boat south in the fall, then spend some time aboard over the winter. She has 200 gallons of fresh water and a pressure shower already. So, if you have a 5, is it enough? Do you wish for more? Or did you install a bigger one and wish you had the space back?