Hello Coach,
As you say your not good with motors, I would suggest you bring it in to Mercury dealer.
By the sounds of it I'm guessing the engine has scored a piston or two. This would be easy enough for the dealer to check out.
The first thing they would do is hook up the laptop and run Mercury's cds program to pull any codes that may have been set.
Then they will pull the spark plugs, inspect them for aluminum and do a leak down test. If they find a reason to, they will pull a cylinder head to inspect.
Sorry it doesn't sound to good.
Marine Mechanic
26 years certified marine mechanic and past state certified teaching degree