Re: silver coating on top spark plug
Certainly sounds like aluminum. This would be from your piston(s). Only on one cylinder, the top one? Few possible reasons, the two most common being overheating and incorrect or over-advanced iginition timing. Also possible you have the incorrect plugs in there which may have a higher heat spec and causing elevated combustion temps.
The fix may or may not require a new piston, and may or may not require any cylinder rework. There are inspection covers on the starboard side of that motor which would allow you to inspect the condition of the piston/rings/and cylinder walls before comitting to a teardown.
Best case is you have an ignition timing issue what has damaged the crown of the piston. The cylinder walls may be just fine. One new piston/rings with a cylinder hone and your back on the water. Obviously you'll want to correct the cause of the damage at the same time. This would be a few hundred dolllars depending on if you are DIY or sending to a service shop etc.
Worst case (unlikely) is you need multiple oversize pistons and cylinder re-bore. The would be a 4-digit $ repair bill.
and if it is how much a ball park range will this set me back $$$. Bore pistons and rings right. New to this boat stuff plz help Thank You.