I owned, fished and floated a number of rivers (and lakes) in the east and west pretty extensively for 20+ years. My first one was a wonderful, if not overbuilt, Kingfisher 11’2” x19” diameter tube cataraft, built by a guy who lived in Sand Point, ID at the time, and I subsequently sold it and got an Outcast 10’ Ferrari Series pontoon which was also excellent. While the Kingfisher was technically a one person boat, I often took a person with me who sat on a cooler that I strapped on the deck.
I know many people are quite happy with smaller pontoons (9’ is quite common), but I’d suggest to determine what size rivers you might be floating, plus what size boat you feel comfortable in handling (back problems?) and then get the very best boat you can afford, with the best oars, anchor system, fins, and manual pump that you can afford, and go for it.
You‘ll find that a lot of the pontoons use on rivers will be to transport yourself from point to point, where you’ll stop to fish likely places, but you can also fish from a moving boat, although it is a bit harder to use a combination of the oars, fins, and your fly rod all at the same time.
Learn how to row and handle the boat first, and stay away from cheap products, and poor customer service companies (of which there are many) and you’ll do fine.
John