I fly a C185 on floats with a 300hp Continental engine. Personaly I think a C172 especialy in those temperatures will just not have enough power to get up on the step without using about 5km of water.
Perfect aircraft would either be a Cessna 180 or 182. Both have a 230hp engine with a CSU and that is what can really help getting out of the water and a 182 that has a turbocharger could be perfect for those high density altitude days.
Hope this helps.