I have owned several production boats with fiberglass (gelcoated) tanks. In the boat we built all water tanks were fiberglass and epoxy coated. No problem. You have a choice to make plywood outsides or to lay the tank up against the hull. Any plywood you use would be best marine grade and saturated with epoxy. If you choose to lay the tank up against the hull, or with plywood, all corners should be filleted with medium density foam, to give radius corners and avoid hard spots. I wouild lay up the tank with at least 3 layers of 6 oz cloth onto the hull, and wrapped up over the plwyood aft end. I would put in fillets on the top of where the wooden top will be secured. Lap the glass over the top of the fillets. Athwartships plywood "bulkheads" should have 1/2" foam fillets between the hull and plywood. The outside of the plywood should have tabbing with layers of glass--starting with 3" than 4" and 6" widths of cloth to spread out any load. The same with apply to the top when it is set in place. When you have the glassed, fair it with epoxy mixed with cabasol and high density filler. Put another coat of epoxy over the faired areas. Put the epoxy saturated top on with a paste of filler and cabasol like peanut butter. Then tape it in place as above. Put the filler, vent and draw tube in the top. Put in an access plate, (epoxy coat the sides of the hole for the access plate, and glass and fair the tank side of the top. You will probably put in fiberglass baffels. I just laid up 4 layers of glass and roving on a relase agent and made the baffels out of fiberglass/epoxy sheets, tabbed in before the fairing. Hope that helps. Bob Austin