If you're going to be running it anyway, you can try something like Seafoam in the gas. It MIGHT dissolve some obstructions in the carb while you're using it, but I certainly wouldn't count on it.
The ideal/easy way to set up the idle mixture is to run the engine at idle in gear in the water (boat strapped to the dock). Let it warm up good, then screw the mixture screw in while idling in gear until the idle speed starts dropping or it starts to miss. Then back the screw out about 1/8 of a turn, which should get it back to "smooth" and the idle speed back up to normal. You might have to re-adjust the idle speed if it's already too low or it gets too high, then repeat the mixture adjustment again.
However, if your engine is misfiring and not running smooth to begin with, it might be tricky to set the mixture properly that way.