I own a Water Skeeter River Tamer II, I have also used several other boats borrowd from friends. I use the pontoon for rivers and have enjoyed water up to class III. While I would agree that a river kayak can paddle upstream with less effort than the pontoon, it's kind of tough to play and land a fish while paddeling a kayak.

With swim fins I can float most sections of the river with my hands free for fishing, I only use the oars for rapids or moving against current. I added a casting platform to my boat and can park in an eddie and cast from a standing position. (Lets see you do that on a yak!)

I can carry a cooler, spare oar, fishing gear, pump and even a small tent and sleeping bag for overnights. Sometime I even carry extra gear for a buddy of mine that likes to kayak. I can carry 2 boats on the top of my subaru fully assembled, or broken down they fit easily into two large duffel bags.

Wind is an issue with small light pontoon boats, a stiff breeze will make for a lot of hard work. An anchor helps keep you in one spot when parked. Swim fins are a must for keeping your heading while hands are busy fishing, with out them you will spin in the slightest change of wind or current.

For moving water I would recommend at least a 9' long pontoon, smaller ones are too easily flipped. Some boats have multiple air chambers in each pontoon, a nice safety margin for bigger water. An aluminum frame is great for keeping the weight down and it doesn't rust. River gear tends to take a beating, so durability will count in the long run.

There are several boats in the $300 price range, but you probally want to look at some in the $600 - $800 range. The more expensive ones are lighter and more durable. That could be important if you end up trusting your life to them. Higher end models have larger diameter aluminum tubing, pontoons made of Hypolon (a white water raft material) or bladders made of urethane. Some of the rivers in my area require at least three floatation chambers. The lower priced boats have steel frames and PVC bladders.