Long Bay is an area that is accessed later in the game, during the final sequences, when Edward tries to locate the Observatory. Once the area is unlocked, it can be visited to retrieve 14 AC4 collectibles, or to solve 1 Mayan Puzzle.
The treasure map you will find on Isla Providencia also leads to Long Bay.
Different from other major locations in AC 4, Long Bay is inhabited by natives. Most of your enemies are hidden in bushes and they are hard to spot, unless you activate Edward’s eagle vision.
While trying to find all collectibles in Long Bay, make sure you scout all areas and deal with your enemies before they see you.
The following guide includes a customized map for Long Bay, as well as a video walkthrough showing how to get all collectibles in the area.