Another thing I have ran into a few times is the throttle not being completely in the neutral position. Looked like it was in neutral, but just moving it a little allowed the engine to crank.
As for stopping to help other boaters, I have towed in several vessels of different descriptions and usually been offered some money but never took any. It may be me next time. If I actually witnessed a wreck, no way I don't go over and see if I can help.