- a line marking the limit of the tide
- any prominent object on shore, as a lighthouse, serving as a guide for ships
- A landmark visible from the sea, used as a guide in navigation.
- A buoy or channel marker.
- A transient natural feature, such as a clump of floating vegetation or a flock of birds in flight, that gives a rough indication of the proximity or direction of land, used as a navigational guide in some traditional seafaring cultures.