I fish from the pontoon boat on the Yakima all the time. It's a great river to float and fish also very forgiving if you haven't had much time on oars. Good place to learn, at least in the summer when the water is high. You can more safely get a feeling for what a pontoon boat will/won't do.
That said, I wouldn't recommend fishing steelhead or salmon from a pontoon boat on the larger rivers. Larger rivers generally have faster flow in them. It may sound kinda funny but at 6 mph hour things happen pretty fast especially when you're trying to manage a fish and keep your mind on things like the snag pile up ahead. So it's not a good idea to fish from the pontoon boat on larger water.
Another thing, and this can't be emphasized enough. DON'T anchor in faster water with a pontoon boat, DON'T. I've cut an anchor off because of the effect it has. Usually the anchor point is about 18" off the water surface to hold the anchor up out of the water when not in use. The higher fulcrum point causes a porposing action from the water sliding under the round pontoons when the anchor grabs, which is apparently quite funny to watch but not at all what you want to experience!
I'd recommend 9' you can get by with 8' but 9' is a little more stable. for the frame either powder coated aluminum or stainless steel.
For the money I think the Bucks Bronco Extreme is probably one of the best out there. You can pick up a bargain variety but eventually you'll wish you hadn't. I think the Skookum products are pretty good too from what I've seen of them.
Look out for boats that are made from the cheaper painted steel frames. The frame rusts and besides you're trusting your life to this thing on the river.
my .02
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Mark Strand
aka - TC