Is it around the spark plug itself or is it around the silver circle that the spark plug fit's into? If it's the silver circle around the spark plug, than it's probably water leaking through the head cover o-ring gasket/seal. Take off the red head cover and you'll see a rubber o-ring in the groove of the head cover hole. Take it out, clean out the groove and put a new o-ring inplace...should do the trick if that's where it's coming from.