I have an 06' Ultra 24' Stealth with a Merc. 496 mag/Bravo 1. The boat only has a 110 well maintained hours on it and operated in fresh water only. I posted a question about the starter a while back, but found that was not the problem, it ended up being that the starter took out 3 teeth on the flywheel. I took the starter to a reputable rebuild shop only to find out there was nothing wrong with it. My questions are these: Is this a common problem with this application? Are there better flywheels recommended? Do you think it's worth a call to Merc.? Out of Warranty of course. Any advice would be greatly appreciated.
Replace the ring gear on the flywheel and the starter with a genuine Merc starter. That will end your problems.
As far as why it happened... Can't tell you. Almost every time we see this it is because someone replaced the starter with an aftermarket starter. It also could be a number of different things. Could have filled a cylinder or two with water at one time and when it hydrolocked it damaged the teeth trying to turn it over and they finally failed.
Bottom line, obviously you need to replace the ring gear. Use a genuine GM part. Even if they say the starter is good, replace it. There is a reason the ring gear is damaged and the starter is usually the culprit. If you don't and the starter has any damage at all to the gear, it will damage the new ring gear.