Assuming you are looking for something that will get the job done, at a reasonable cost, I would go for the Rinda Mercruiser scan tool. http://216.230.203.183/marine/mtspec.htm
I haven't looked for a year or two... according to Don's post, they must cost more new. I bought mine for $250 off Ebay with some extra ECM adapters. If your patient, I've seen them go for less that that.
You can set up your Ebay account to Email you whenever a "Mercruiser Scan Tool" is listed, that's how I got mine.