Ranger Militia are non-combat units that are granted upon building any oar and sail powered naval units, such as a Galley or Galleon. They automatically appear on board the vessel when it is complete; however, they also have the ability to scout on land. This makes them useful for discovering artifacts on isolated islands.
Ranger Militia have an attack of 0, a defense of 1 and a movement of 1. A naval unit carrying a Ranger Militia that sails adjacent to another civilization's city will reveal the unit defending the city. Although they cannot be ordered to attack any opponents, the militia can be moved like all other ground units.
The Ranger Militia can also be acquired by capturing a naval unit from a Barbarian village. This unit, or any other units on a ship, cannot be captured, they are lost with the vessel. Ranger Militia units on board vessels that are upgraded to Cruisers by the completion of Leonardo's Workshop are retained.
If possible, land the Ranger Militia off a vessel before it is decommissioned (a Galley facing imminent destruction by a superior force, for instance). It will not be removed when the ship is decommissioned, and the same amount of gold will be received. The militia unit can then be either used elsewhere or decommissioned for an extra 5 gold.