My dealer told me that the cavitation plate on the motor should be about water level when running at cruising speed. Mine was initially low so I raised it an inch. it was easy to do the Steve's jackplate but after doing that, I seemed to get a lot of what sounded like a combination of exhaust and cavitation noise so I lowered it back down where it was and the noise went away. I didn't notice a difference in performance at all, but then I'm not trying to get 70MPH out of a bass boat.
The important thing is to get the motor to run at the max rated RPM with the boat loaded like it normally is. Then you can get the prop worked if you need to, but with a utility fishing boat, that probably isn't going to be worth the $$$ to try to tweak it.
2015 Seaark 1872 MV CC
Etec 90 HP
Bitter Gun Owner