Alternative: Thalatta, Thalath
Thalassa is the second moon of Neptune. Thalassa was named after a daughter of Aether and Hemera from Greek mythology. "Thalassa" is also the Greek word for "sea". It was discovered in 1989 by Voyager 2. It is irregularly shaped and shows no sign of any geological modification. Since its orbit is below Neptune's synchronous orbit radius it is slowly decaying due to tidal forces and will one day break up into a planetary ring or impact on Neptune's surface.
Thalassa is a fictional planet, a waterworld in Arthur C. Clarke's novel Songs of Distant Earth.