The USS Mare Tranquillitatis was a 24th century Federation starship operated by Starfleet.
The Mare Tranquillitatis was commanded by Captain Douglas Drexler. Keith Rocha was posted to this ship as an operations officer upon graduation from the Starfleet Academy. This posting was named in the personnel status report for Lieutenant Rocha. (TNG: "Aquiel", display graphic)
This starship was only mentioned in writing.
Mare Tranquillitatis ("Sea of Tranquillity") was a location on Luna. In 1969, the lunar module of Apollo 11 landed here.