The aquatic setting allows for a completely new dynamic — even if both films take their stylized look from the original graphic novel by Frank Miller.
Empire "is tied visually to the original,'' says director Noam Murro, but with so much happening within small boats, "there is a whole different choreography of fighting and war.''
The sea battle also allows the scale of the movie to change. The first 300 took place primarily on a narrow pass known as the "Hot Gates," where the Spartans sought tactical advantage due to their significantly smaller numbers. But Empire "happens over time in many different locations," says Murro. "The opportunities for the six distinct battles are even greater with different locations and tactics.''
http://www.usatoday.com/story/life/movies/2013/04/08/300-rise-of-an-empire-sea/2064101/