boat engine surging

Surging at Part-Throttle is gonna be an AFR problem.
Could be in the carb, or could be an air-leak into the crankcase. But on a 2-cycle could also be in the ringseal, as in rings or cylinders.
Because the idle is affected intermitantly, I'm gonna go with a vacuum leak; or, as mentioned the fuel pump or it's signal line.
And don't forget the rest of the supply line. Primer-bulb connections are a known trouble-source.