Propane has approx 95K BTU energy per gallon, gasoline approx 135K BTU per gallon. Seeing those cylinders are about a quart, and they are like $3 each, it would be quite expensive to run it much and your range would be greatly reduced.
Propane is a bit cleaner though and as long as its a 4 cycle engine its probably somewhat of a wash. But seems more like a fad or some feelly good green thing more than something truly practical, to me anyway.
If I were really only interested in getting out of the marina and back to port, a large trolling motor would make far greater sense. I dont know if minnkota stills makes them but they used to make a 3 HP electric trolling motor. If one isnt enough, mount two. There are also marine electric propulsion drives for mini subs and the like that could be adapted to fit virtually any boat.