I spent some time on the phone with a couple of Merc dealer techs, and got a few responses. Base gasket turned to mush, exhaust carbon build up/tarring, carb clogged, coil cracked/broken and on and on.
Some times it's really simple though. I figured I'd change the plugs and test spark, and while doing that I went back and forth a few times from cylinder to cylinder and noticed one with lower spark. Tried swapping plug wires from one to the other and while doing that I felt a "soft" spot in the longer plug wire. Cut the wire at the soft spot and there it wasn't! The plug wire had completely corroded through at that spot. I pulled the coil end crimp off, moved it to good side of the wire, plugged it back in and next thing I know I'm doing 20 mph +/-!
I figured I'd post up the solution here as when I was researching this, 90% or more of the threads I read the OP never posted what his outcome was. It's hard enough to diagnose things over the web, but even harder when you read a bunch of problems but don't see any results. I wouldn't have thought my engine was only running on one cylinder because it started so easily and ran so smoothly! Hope this helps somebody else.