Not sure if it's applicable to your proposed heater installation, but the Espar Airtronic system on my boat has a through-hull fitting as a means of exhausting combusted diesel gases and looks similar to this image:
Very unobstrusive and double-walled for thermal isolation against the GRP. It's mounted high on the port aft side of my double-ender, just below the toerail - approx. 60 inches from the boot stripe. At the interior side, the stainless steel, corrugated exhaust hose incorporates a high loop, preventing water intrusion - although that would require a heeling angle close to a knockdown.
Typically though, the vent is mounted on transoms of sailboats.