Sea mark

There are three types of sea mark: beacons fixed to the seabed or on shore, towers (sometimes rock cairns) built on the shore or on a submerged rock/object (especially in calmer waters), and buoys (consisting of a floating object that is usually anchored to a specific location on the bottom of the sea or to a submerged object).

Sea marks are used to indicate channels, dangerous rocks or shoals, mooring positions, areas of speed limits, traffic separation schemes, submerged shipwrecks, and for a variety of other navigational purposes.

In a wider sense the phrase "sea mark" is often understood to include all types of landmarks, structures and devices that can be used to provide warning and guiding signs to mariners.

When approaching a harbour from seaward, Region A places conical green marks to starboard and cylindrical red ones to port.

Additionally, since marks may be off station due to collision with ships, storms or other factors, they should be used with caution and not relied on solely for navigation.