You'll need to side with either Aquaman and his loyal Atlanteans or Ocean Master and his rebellious Mutineers. New content includes a four-player boss battle, open-world missions on the water between Metropolis and Little Bohemia, and plenty of new rewards and item drops.
SOE says that Tides of War is available "for a limited time," but the firm isn't being specific on just how limited. Long story short, check it out while you can!
[Source: SOE press release]