A museum ship, also called a memorial ship, is a ship that has been preserved and converted into a museum open to the public for educational or memorial purposes. Some are also used for training and recruitment purposes, mostly for the small number of museum ships that are still operational and thus capable of regular movement.

A lightvessel, or lightship, is a ship that acts as a lighthouse. They are used in waters that are too deep or otherwise unsuitable for lighthouse construction. Although some records exist of fire beacons being placed on ships in Roman times, the first modern lightvessel was off the Nore sandbank at the mouth of the River Thames in London, England, placed there by its inventor Robert Hamblin in 1734. The type has become largely obsolete; lighthouses replaced some stations as the construction techniques for lighthouses advanced, while large, automated buoys replaced others.