On I/O's, especially the new ones, the shaft the prop is connected to, is supposed to break loose for this exact reason. You can go to your local dealer (not necessarily the one you purchased your boat from, just a dealer that works on Mercruiser engines). Make sure you take the SN with you, and tell them the problem. They should have a replacement handy. I have found that all the places around here have the parts for the same price as Mercruisers website. I have found some things at certain sites cheaper, but then you have to pay for shipping, and then wait.
Anyway your dealer should be able to get you the part. Should be easy to replace.