I have used a Watermaster on streams many times. The raft part is very well built and durable. The raft is also very sea worthy.
The oar system is the weak link. For starters they are too short contributing to the crafts slowness. Secondly they are not very sturdy and the oar lock system is subject to breaking frequently. Be sure to carry spare oar lock parts.
The handling characteristics were well described by the preceding responders and contrasted to the pontoons well.
In summary, I prefer the pontoon for rivers.