Hi there, welcome to iboats.
If possible on your end, try posting your Volvo engine serial number to see if it may just help one of the Volvo dealers that tend to hang out here to see if there happens to a service bulletin regarding something like this....Long shot but you never know due to the long possible list that your issue could be as I'll try to describe below for a starting point for you.
What cylinder (or cylinders?) are being affected?
What does the other spark plugs look like?
What does the engine temperature get up to and how long does it take to warm up?
Is the engine still under warranty?
How do you use the boat? Mostly at idle RPM's for trolling for fish, etc. or high speed runs or cruising speeds?
If any, what does the dealer say thus far?
Most important question, Does the spark plug seem to be fuel fouled or oil fouled mostly???
Being a direct injected engine, maybe one of the injectors are being left open to long from maybe, debris in the injector/fuel system/tank, etc., an ECM programming issues, to much fuel psi, or possibly an ignition issue such as lower 12v and 5 volt power and ground problems, bad spark plug wire/s or wire boot going to ground, one of ignition coils failing, incorrect spark plugs or plug gap.
Any one of the sensors could affect this as well if they just happen to be on the not so normal range but yet not to far off range wise to cause a sensor fault code to go off usually picked up on a scan tool.
Being catalyst exhaust, possibly one of the oxygen sensors are just out of normal range changing fuel flow rates, incorrect engine oil for these exhausts plays a small part in all this too....Quick tip, Don't let the engine run out of fuel either as that can be damaging to the exhaust too.. $$$.
Could even be an internal engine mechanical failure or component design issue happening....It's sometimes a big list to go through to find issues such as these. Plus being intermittent makes things even more challenging at times.
Hope this one will be an easy one to find for you.:encouragement:
