Fewski,
You have chosen a great aircraft. The Avistar is my recommendation for a beginner aircraft. Now lets see, you want to fly off water. I won't discourage you, but as a beginner you will not only be facing the challenge of learning to fly, but you will also be challenged by H2O. A float plane will require a lot of power on a clam day, as there will be no wave action and suction builds under the floats. This will require a long take off with a 40 size motor. The 46 size motor will be an improvement, but may still not be enough. A 61 2 stroke would be perfect, but may be too big fot the fire wall. I recommend you get the best 46 2 stroke you can afford. Be patient when taking off, as with the added weight of floats, not to mention the "Drag" effect, a take off will be long and progressive. I also recommend you purchase a flight sim and practice until you can take off, fly and land without crashing every time, then proceed with your trainer. I too recommend you seek help from an experienced R/C Aviator before going on your own. Good Luck and Happy Landings.