Fishing from a pontoon on a river is just fine. Be sure to understand the trespass laws of the stream being fished. Some states like MT pretty much give you free range to anchor up in the water or beach your pontoon for some wade fishing as long as you stay below the high water mark.

Others, like WY, are much more restrictive in that the land owner has title to the river bottom and any anchoring, beaching, or wade fishing can be prohibited.