A maritime pilot, or simply pilot, is a mariner who has specific knowledge of an often dangerous or congested waterway, such as harbors or river mouths. Maritime pilots know local details such as depth, currents, and hazards. They board and temporarily join the crew to safely guide the ship's passage, so they must also have expertise in handling ships of all types and sizes. Obtaining the title "maritime pilot" requires being licensed or authorised by a recognised pilotage authority.