I got Cutthroat I tubes from Jack's Plastic Welding http://jpwinc.com
that are great for tight channels and low water. (Also fun for tight channels and high
The tubes are about 12 ft. x 16 inches. I ordered them with 4 d-rings per side and top chafes. For low water with a load, double bottoms might be good, too. The frame is homebuilt of 3/4" ips aluminum pipe and SpeedRail fittings. It handles like a Mini Cooper.
Jack sells the tubes with a frame and you can also get spreader bars to make it wider— I just like building frames. The tubes have enough waterline that they track straight in the rough stuff (smaller fishing cats with rockered tubes have a tendency to spin and flip).
Here it is rigged for gear, beer, and pooch on a 3-day North Platte trip. I weigh 225 and the dog about 60, plus all the camp gear and food. Riding a touch low, p'raps.
One nice thing about the 16-inch Cutthroat tubes is that you can get a Fat Pack Cat frame and have a paddle cat as well. This is the 16-inch Fat Pack Cat I rode for 15 days on the Grand (shorter tubes with different rear cones).
For narrow, rocky channels like the Escalante in Utah, the Pack Cat is better because the width of the Cutthroat with oars is awkward. The load capacity is obviously the same.
Anyhow, hope you find a boat you like.