I use one of these in my ice fishing shanty.
I would question the safety of having open flame in a boat due to possibility of gas fumes. You wouldn't see the flame in the daylight but there is a light rolling flame from time to time across the ceramic.
This heater has a neat safety feature. If only slightly bumped, it will shut down. Also if a puff of breeze hits it it will shut down.
Works great inside a ice shack.
If you could feel the heat 4' away in an open boat I would be shocked. I can put a pair of damp gloves within an inch of the guard grill on the front for a short time in the ice shack. ole Mike