Dolphins are marine structures that extend above the water level and are not connected to the shore. They are usually installed to provide a fixed structure when it would be impractical to extend the shore to provide a dry access facility, for example, when ships (or the number of ships expected) are greater than the length of the berth.

Typical uses include extending a berth (a berthing dolphin) or providing a point to moor to (a mooring dolphin). Dolphins are also used to display regulatory information like speed limits etc., other information like advertising or directions and navigation information like a day beacon as well as ranges and lighted aids to navigation.

Mooring dolphins can also be used to "cushion" ship impacts, somewhat similar to fenders. The structures typically consist of a number of tubular steel piles driven into the seabed or riverbed and connected above the water level to provide a platform or fixing point. Larger dolphins would typically be fixed using a reinforced concrete capping or a structural steel frame. Access to a dolphin may be via a pedestrian bridge (mooring dolphins) but is usually by boat