The largest new region is the Shattered Empire, a once prosperous collection of islands that's now haunted by ghost ships. It has more than one mini story to pick up—you can try to lift the ghostly curse, save a plague-ridden colony, and rebuild (or destroy) a monument to the Empire.
In the Forsaken Seas, the second new region, the air is heavy with poisonous gas, which will slowly eat away at the health bars of both your and your enemy's crew, changing how you approach combat. The last new zone, the Howling Seas, will see your ship battered by blizzards. During combat, those blizzards can stun your crew, and even knock them overboard.
All of the new regions have a unique island to discover and a unique side quest, which developer Fireblade Software says will be the case for all areas of the game in future.
As well as the new regions, the update makes navigation easier by adding lighthouses in every zone which, when visited, will dispel the fog of war. You'll know where the lighthouse is from the start, so you can make a beeline for it if you want. Your ship also now has a compass that will point you towards the nearest undiscovered point of interest, which should make your travels feel less random.
Lastly, the update lets you make both friends and enemies out at sea, depending on whether you help or hinder strangers you come across, and there's a chance you'll see them later in your playthrough. Your actions will have consequences, so be careful who you wrong.
The full patch notes are here, if you're interested.