The new map, called Nassau, is apparently the new most difficult area in the game. It features ten new secondary missions and 12 additional treasures. There's also an extra campaign mission and three more ships.
It seems gameplay has been tweaked somewhat as well—the new App Store description reads, "Put your defense skills to the test in new defense gameplay: use new strategies, like destroying cannonballs in the air, to turn the situation around by counterattacking your enemies!" Supposedly the difficulty has been increased across the entire game, too.
Check out our Assassin's Creed Pirates review for more.