OK, to start with one that might not be so obvious but I did it when running both inboard and outboard engines. Did you reverse the wires for the starter?
You have to watch 20% nitro as generally it doesn't contain enough oil for boat engines unless you use Heli fuel which does run 20% oil.
Yes, it could be the wrong plug doing it also as I've found out in the past. I used the MC-9's in my nitro days as they seemed to work well in anything from .12 to .45, the largest size I ran.