Larry,
IF you understand that it's the gasoline vapors that ignite under certain conditions and IF you keep your flame (or spark) away from those vapors, then you should have no worries. My biggest concern about using a heater in a boat is flammability of gasoline vapors. It's the vapors that ignite when they're above a certain concentration (lower flammable limit) and a flame or spark is applied. Gasoline vapors are heavier than air and will tend to settle on the floor in a closed space that isn't vented. Take precautions to prevent build up of vapors in the boat and keep the flame away from the vapors, and you should have no worries. Also remember that vapors can be present even if you don't smell them.
I'd be less concerned about sufficating from carbon dioxide build up as aresult of using a boat heater, than i would about flammablility. as long as you're vented, you should have enough air to breath.
be safe and enjoy your hunts.