I agree the pitch is an important consideration when choosing a prop. Ideally the prop will be matched to the motor and model to achieve its designed speed. Easily said but not so easily calculated or as you indicate by trial and error.
Too coarse a pitch will overload the motor, Esc and battery, generate lots of heat and disappoint with lack of performance. Ideally and especially with brushless motors you should use an amp/wattmeter to measure the power being taken at full speed whilst you hold the model stationery in the water. This should be below (50/75%) the max for your motor/Esc and battery. Adjusting the prop pitch, number of blades and size will allow you to keep within these limits.
If you are a competition entrant then I would expect the equipment being used would support much higher specs so my comments are not intended for such situations.
This will indicate the max size and prop pitch for your setup, allowing you to then test various combinations on the water to see which suits your model. I have an on board sensor that reports individual cell voltages back to my tx. I used this method with my brushless Sea Queen and obtained good results. Treated myself to a 3 blade brass prop from Prop Shop and the performance improved even more as well as using less power.
Weight can also have a marked effect on performance and just like my planes from my flying days, the lighter the better.
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