If a spark plug comes out with rusted tip there's a water intrusion going on in that cylinder, mainly due to a bad cylinder head gasket, exhaust cover gaskets if having them or a combo of both. Retorque the whole powerhead to factory specs, if keeps rusting that same plug change both gaskets for new ones, clean well all mating surfaces beforehand and torque to specs, retorque after first 5-10 run hours.
Happy Boating.