Help! Was back flushing water intake hose and got out of the boat to see how the water was coming out. Noticed a stream of water coming out of where the front nut/stud from lower unit bolts to the upper unit. Got to looking and found a round hole just behind the grounding plate. Is that supposed to be there (open) or should it be plugged w/bolt? Could not any reference to this hole in my manual. Was going to take the boat out on Lake Michigan, but not sure if I should until I know what's what. Any help would be greatly appreciated.