Why not a C172 on floats? Not sure if amphibs are available though. Piper Cubs & C152s can also be float equipped but not sure if they can handle the extra weight of amphibs.
I did my float rating in Florida in a C180 on amphibs. A/c is based at an airport so maintenance & fueling is easy. The airport has a designated landing area on an adjacent lake + there are any number of other lakes in the area.