Thalassa (tha LAHS sah) is one of the oldest names I can think of. It is Greek, meaning "sea, ocean." Thalassa was the primordial spirit of the sea, daughter of Aether and Hemera, thought to be the mother of Aphrodite. She was married to Pontus and mother of the Telkhines, sea creatures with webbed hands but the heads of dogs. (Leave it to the ancient Greek imagination.) She was not really a goddess, but the personification of the sea, the creator of all sea life, thus the Mother of the Sea, and people around the Mediterranean believed she was the sea itself.
You can see a recent installation of her likeness in the New Orleans Museum of Art, which is the most recent tribute to her, despite how ancient the goddess is. A moon of Neptune was named Thalassa, as well as a fictional planet, a genus of ladybug, a poem, a band, a TV series, a book, a restaurant, and a ship. The mercantile sea kingdom Thalassocracy was also named for her. I also wonder if she is the mother of the main character in Ponyo on the Cliff by the Sea, the recent Studio Ghibli animated film.
Thalassa was around a short time before the other gods and goddesses we know more about, like Athena and Apollo. It makes me upset that other baby name sites don't provide this information about her, and often they will say the meaning of Thalassa is "sea goddess." Nonetheless, Thalassa is simply a gorgeous, deep, mysterious, alluring name. If you wanted, you could nickname her Tally, Thallie, Thalia, Lassa, or Lassie. There were zero or less than five baby girls with the name Thalassa born in 2010.