I want to get a jet boat for a small shallow river I will be fishing often in Missouri. Problem is, I dont know where to start. I looked it up and all I found was a river pro boat. It was pretty nice, and had the length (18,5 feet) and all the features right, but it was very expensive and had a big motor (175 hp). I only need about a 40 hp motor. Any suggestions?

Try doing a search on Missouri boat dealers.  In specific, check into the dealers in Steelville, Cuba, Sullivan, St James, Salem, Rolla.  A lot of those dealers deal heavily in boats built specifically for river running.  They often custom build.  Remember,  a jet drive is relatively inefficient for translating engine HP into straight power.  A 40hp motor will perform at around 1/2 to 2/3 like a normal prop driven 40hp.  

I know little about this subject so I'm mostly talking out of my arse but can you find a shallow draft, aluminum boat that will do the trick?  How shallow do you need to go?

A shallow aluminum boat doesnt have the features. A jet boat is perfect here because it has the features of a bass boat, but it runs much more shallower than a bass boat.

I have owned 2  jons with jet drive outboards.

Both of my boats were made by Lowe boats. First one was a 1652VPT, second was an 1860VPT.

I would look at a Lowe Roughneck or utility jons. I currently have a Tracker Grizzly 1648 and I'm searching for a tiller jet drive o/b around 30 hp.

Here's a link to a local dealer here. Basically, all he deals in are jet style boats. If you call you will get all the answers you need!

www.wiegelbrothersmarine.com

Yamaha offers jet drive outboards in 40, 60, 90, & 115 HP

Mercury offers jet drive outboards in 25, & 40 HP

I have a 16' Lowe with a 60 yamaha jet and would not go any lower in horse power. Like someone said you loose about 1/3 or a little more of your horse power when you put the pump on the bottom. I can go any where I want with two people and all our gear in about 5" of water--as long as you don't find that one rock that got push up from high water With 3 people it is more like 6-7" of water.

Doug