The Huron Belle...A Pilot Boat

If you look just north of Port Huron, you will sometimes see salties at anchor.

 Typically those salties are waiting for a pilot.  There are only a certain number of Great Lakes qualified pilots out there and the salties generally don't have a permanent one on them.  I think there are designated transfer points.

Her the Huron Belle is taking a pilot out to a waiting ship.