Personally, I think the best seaplane out there is the Seamaster (Or the Neptune which is basically the same plane)

It is not as glamorous as a Cub or Beaver, but a single flying hull is MUCH easier to take off and land with in water than a set of floats.

Also, is it safe to assume that you have a boat on this river? The bad thing about rivers is that if you dunk it, it can be miles downstream if you don't have a boat ready!