Alternator belts on the MerCruiser are designed to last a long time. However, a wise boater will always carry a spare aboard, just in case of trouble. Replacing the alternator belt is a simple task that can be performed by any do-it-yourselfer.
Loosen the bolt on the alternator adjustment slide. This slide acts as the belt tension adjustment. Loosen the alternator mounting bolt. It is not necessary to remove either bolt.
Pull upward sharply on the alternator belt. This will cause the alternator to move in the adjustment slide, making it possible to remove the belt. If the belt is completely broken, you will have to move the alternator in the slide by hand.
Put the new belt in place on the pulleys. The "V" shape of the belt faces downward on the pulleys.
Slide the alternator back to its original position to take up the slack in the belt. While holding tension on the belt, tighten the slide and alternator bolts that were loosened earlier. Proper tension is checked by moving the belt up and down with your fingers. This check should be performed halfway between the water pump and alternator pulleys. There should be ½ inch overall travel when moved with slight pressure.