Products of combustion (when perfect) are CO2 and H20 (water). Any non-vented combustion heater will be adding water
vapor to the inside of your boat. Assuming its cold outside (duh), the cold surfaces of the boat will condense the moisture and you will soon be all wet and moldy.
heat does not generate any moisture and does not need to be vented to the outside. Of course, will likely need to run generator
Good option if plugged in at the dock
Good propane and diesel
heaters have products of combustion vented to outside to remove moisture, CO2 and any CO (if present). Need to make sure you have sufficent source of makeup air to replace the air vented to the outside by the unit (open dorade vents or a partially open port somewhere). Also need to be careful where opening is: A open port on the lee side of boat in a breeze can cause the boat's interior
to go to a negative pressure and back-draft the products of combustion from the flue (air comes in flue stack instead of going out of it). This can be a serious safety