This engine on a boat may not seem to have a huge advantage, but with further insight it holds many advantages.
The best application of course would be for house boats with lots of surface area. The solar ability could power the boat and energy store could carry it through the night. Solar trolling could also be enabled, especially on Lake Powell and remote lakes where fuel is the limiting factor.
The fuel advantage in the cruise would be 2.4 time over a gasoline engine (60%/25%= 2.4) and 1.7 over a good diesel (60/35=1.7).
Where the engine shines is low power, micro jetting for vectored thrust via water jetting, and typical house water processing issues such as shower water, drinking water and waste that consume lots of fuel.
The engine can be placed in the boat where the design needs it to be rather than where the engine needs to be due to constraints. The peak power ability of the engine allows it to come on step but plane efficiently. This feature is ideal for water ski boats that need a burst of power and then efficient cruise.