Its on the starboard engine, that runs the fuel pump/impeller, power steering pump, and alternator. any idea what size that belt is? 2000 carb'd 500hp

If you don't have the old belt, then just take a strong string or thin rope, set the adjustment for a little more than half way in, and wrap the string around the pulleys. Tie a knot and then see if it will get tight with the available adjustment. Then move the adjustment all the way in and remove the string.

Take that to the auto parts store and match it up with a new belt.
Kent