This plane, and boat, guy says go with the Tower.

For props, the manual suggests breaking it in with a 12x6 prop and says a 13x6 could also be used. Many of the 3 blade recommendations I've seen suggest stepping down 1 inch. This would mean an 11x6 or 12x6 3 blade prop should work. Keep in mind that another way to reduce the diameter is to go up in pitch. Going down 1 inch in diameter, but up one inch in pitch gives approximately the same load on the engine. So, a 2 blade 11x7 or 11x8 might give similar results. The increased pitch will possibly give you higher speeds. The reason I say possibly is that by moving less air, but doing it at a higher speed, they may not be able to overcome the drag from the water as well as a larger diameter, but smaller pitch. Experimentation will be needed to find the best prop. As you're doing this experimentation, it would be a good idea to buy, or borrow, a tach so you can measure the RPM range of each prop. This will help you ensure that you're not over or under propping the engine.