Have a look at the jet boat web sites in New Zealand,(www.nzjba.org
) true river boating was pioneered here. I have a 15' Hamilton (www.hamjet.co.nz
)152 hull with a 302 ford driving a 3 stage 773 Hamilton pump, it will go anywhere you have 2" of water or more, can pay anything from a few thousand to fifty thousand for a boat. Don't waste your time with PWC jet drives as they are far to small to push a 15' boat. Mercury Sport jets are gaining favor as they are light and easier to push off the rocks than a boat like mine. Anything around a 16deg dead-rise at the transom gives good shallow water capabilities but may pound a little in big water.
I did some boating in a little Hamilton 131 (14') with a 120 sport jet a few years back and it went great, only needed half a bucket of water to get up and going and could plane on a wet rock, I asume a 90 would be great also.
Rip into it you'll love it.