Since there seemed to be more interest in that project than I expected, I spontaneously decided to upload my files (also step and f3d) here.
If you want to see the model in action, I made a short video:
(!) However, please take the model with a grain of salt, as the light construction might make it fragile so it might require foam inside the floats on each side. Also, the motor holder angle might be too big in the original .stl (!)
Anyway, as electronics and parts go:
I took Motor, ESC and Radio from my E-Revo VXL Truck, so about 3000+ KV and 30+ Amps ESC; combined with a 3 Cell Lipo from Turnigy Nanotech, 2200 mah and 45C+ discharge rate; standard 9g servo, waterproofed with hot glue and grease... The motor is connected to a 3mm flexible stuffing tube (that I salvaged from a broken HK boat) and runs a 27 (or 32)mm nylon prop (don't really know the pitch, but it's probably something around 1.5 inches per revolution).
Hope you will have fun nonetheless with my design, feel free to edit and improve it (step and f3d should be included)