Likely need new mech who doesn't want a new boat before summer ends. Plug covered in oil will not fire can have many causes, did mech use new plug in an check for spark?
Address misfire first then worry about other code if it comes back after being cleared.
X2 on what is being put to you from your mechanic.
The code is a misfire.
Fouled plug ? No spark, no compression, burning oil, burning coolant...... Many possible causes but not likely all
I think your mechanic is either trying to get a carte blanch repair OR is trying to scare you away ( probably because he does not know what he is dealing with????)
You need to get more information! You need to ask the right questions to get them
Ask him " does it have spark?
If no you have an electrical issue such as a coil or bad wire.
Does it have compression?
If no you have a mechanical issue such as a broken rocker arm
Is it leaking coolant? Is it leaking oil?
The more solid answers you can give us , the more we can try to help.