Where is the water-line going to be? How much of the propeller is going to be submerged and even, what size and pitch are they? You don't want the props to cause any cavitation by one, too fast an RPM, too small in size and not deep enough in the water.
Just some food for thought. I had a few boats I built that had those problems to overcome and let me tell you, it would have been easier to buy an RTR but I stuck with it and had the fastest boats on the water using very few battery time. I owed it all to this ugly mechanical speed controller that cost about $100 but was the cream of the crop in SC's!
~ Jeff