Shawn; here's a video you might enjoy... if you want to build one for yourself watch all the vids from 1 through 10 to see how it was built. A red neck jet boat.
Basically this is trash pump drive.
On a more serious notion I believe you actually need this
Shawn; it just so happens that I will GIVE YOU a gas powered weed eater (industrial strength with broken blade holder on end of shaft, but it works all good otherwise) if you will make one of these!
This would work awesome in the river; easy to tilt when approach rocks and I'll bet it would push a 14 foot jon boat with ease.
Heck; I would have thrown in the jon boat but it got ripped off from my son's yard several weeks ago... it was Sears "ted Williams" vintage early 70's. Think a couple meth heads stole it for the metal.