And here is a neat way to have a solar propelled boat using the German Torqeedo high-efficiency motor. I suspect in this video the battery is not completely dead but is neat otherwise to have quiet propulsion that goes on forever!
My boat (Lund Pro V) had 101 36 volt electric motors front and back, three big batteries with an inboard charger ... required to move lots of weight. It was designed so that it could maintain stationary positioning (but not anchored) for vertical jigging purposes over deeper water, in lakes which tend to be rough and choppy. Much power is consumed in the process, so the "trickling" of Solar energy wouldn't have helped.