Dave,
look at Pontoon boats with electric motors. My friend Jim Barry has been instaling them on pontoon boats for people that want to convert or the lake is electric only. these electric motors push 16-20 ft pontoon boats i bet they would push a jon boat real well. But there is a price for that too. If you get serious about this i can give you his number.I bet it be cheaper to set up triple bow mount motors. i know a guy that can machine a bracket and probably engineer a way to have them turn simoutanously with one handle.