Red buoy in San Diego Harbor. Note red light, number, radar reflecting corners and seal.

A sea mark is a pilotage aid which identifies the position of a maritime channel, hazard and administrative area to allow boats, ships and seaplanes to navigate safely.

There are two types sea mark:

• posts - fixed to the sea bed
• 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 locate channels, dangerous rocks or shoals, mooring positions, areas of speed limits, traffic separation schemes, submerged shipwrecks, and for a variety of other navigational purposes. Some are only intended to be visible in daylight (daymarks), others have some combination of lights, bells, horns, whistles and radar reflectors to make them usable in all weather conditions. Marks often display a number that is also noted on nautical charts.

The International Association of Lighthouse Authorities defines several systems of marks specifying the shapes, colours and characteristic lights of buoys depending on their purpose. Perhaps the most well known convention is placing red or triangular marks on the side of a channel that would be to starboard as a ship approaches a harbor from the ocean, with green or rectangular marks on the port side. Hence the famous mariner's mnemonic "Red, right, return."

See also: lightvessel, lighthouse, landmark