What we know:
The film centers on Themistocles and Artemisia I of Caria, as well as Xerxes I of Persia. It originally went under the working title of Battle of Artemisium. The Battle of Artemisium was a naval engagement, concurrent with the battle at Thermopylae, and was fought between an alliance of Greek city-states and the Persian Empire in September 480 BC, in the straits between the mainland and the northern tip of the island of Euboea.
And now what you came for. The trailer.