Cardinal refers to the four main points of the compass: North, South, East, and West. In maritime navigation, cardinal marks are a specific type of buoy or beacon that indicates the direction of safe water relative to a hazard. For example, a North cardinal mark means the safe, navigable water lies to the north of the buoy.
Heard on the job
Keep the vessel to the east of that cardinal mark; there's a submerged wreck on the other side.