1. The boxes have an efficiency of about 50%, which means there is a considerable amount of heat dissipated (probably close to that of a diesel engine of similar output).

2. In normal operations, one assumes that the raw fuel and the heat are well separated, yet if you put fuel mixed with air in touch with the core's heat, it will ignite. If that happens in your kitchen, you could run away. In a boat at sea, you are screwed.

3. I'm only saying that while the product looks promising for a plethora of uses, yet it doesn't seem that boating would be one, unless you assume as acceptable the risks of gaseous fuels at high pressure near a thing that reaches 1000 C. Please note that all shown installations are outside buildings.