We had a Coronado 25 on San Francisco bay. Started with a 6HP which was OK for calm waters but at times marginal for inside and outside the bay. Went to a 15HP and it was plenty. Probably a 9.9HP would have been OK.
Suggest you go for a long shaft and stay with the well. Have seen a number of good looking motors on our local Craigslist. If I was to do it over I would get an electric start motor. good luck in your search.
What did you think of your Coronado? 15hp really, I thought it was limited to 10hp? You wouldn't happen to have any pictures of the engine well lying would you? I have to rebuild the engine mount as mine has rotted away.