That is a good question. I would say try a 15 turn motor. I also have the Profisherman. I cracked my hull a couple of months ago by being stupid and seeing how close I could come to the side of the pond where a brick wall was and bam. Cracked the tip of the hull bad. I repaired it and it didn't look too good. So I called Tower to see if they sold the hull seperate as a replacement and they didn't. They gave me the number to their hobby services which is great planes. The guys at hobby services sent me a Free used red hull. Which was very cool because I had the Grey one that I didn't like very much but it was all the LHS had when I bought mine. The outside of the red hull is like new. The inside just had a little mold on the wood.
I have tried a 19 turn motor and it is pretty fast. I have always wanted to try a 15 turn motor in it but have not had the extra money to buy another esc for it. I was going to try a 400 Feigao Brushless motor in it, but before I could try it I sold the 400 Feigao and the Hacker ESC in a Gecko Boat that I sold as a RTR.
So anyway I would think a 15 turn double would be a good place to start. A prop upgrade would also be nice. Ask steve at offshore what would be a good prop.