What kind of heater and where do you plan to mount it........bulkhead, force air, hydronic and are you asking about flue gas exhaust or hot air ducting? Most manufacturers provide elaborate instructions on both flue gas exhaust and hot air ducting installation. For a sailboat I would mount a forced draft diesel heater in the aft lazarette with the flue gas exhaust out the hull above the waterline minimim distance from the heater (refer page 9 in the typical attached installation instructions). My powerboat has one with the flue gas exhaust through the side of the hull immediately adjacent to the heater.