Sea mark
A sea mark, also seamark and navigation mark, is a form of aid to navigation and pilotage that identifies the approximate position of a maritime channel, hazard, or administrative area to allow boats, ships, and seaplanes to navigate safely. 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).