There are at least six words.
Thalatta (now "thalassa") , the sea in general.
Pontos, the sea as a road or means of transport.
Hals, the saltwater, from which we also get Halyia
Pelagos, meaning the deep sea. Also Archipelagos, the primary deep sea.
Lastly Okeanos, the Ocean Sea.
The three Ancient Greek words for "sea" that Julieta writes on the blackboard during class, are "thálassa", "háls" and "póntos".