This motor is based on the GM Vortec 5.7, correct?

I know many things are different to make it sea worthy and then throw in the PP system and it further changes. However, the motor itself does not appear to be the problem, but we can rule that out if you would like.

Can you pull the throttle cable separately from under the sun pad while in neutral? Do this after it malfunctions but before restarting. It should rev up. Just making sure it is the PP system and not a TPS issue on the motor.

Next would be restarting and setting the PP and seeing if it holds true. If the speed runs away or struggles keeping 15mph then start with the boat speed sensor and check for wire rub and corrosion.