I posted this to reddit yesterday but here is what I wrote

I'm not sure if I've found a potential skip or not, but let me explain what happened:

During the mission Vainglorious Bastards, Sequence 08 Memory 02, during the fight with the African Co. Frigate and the 2 brigs and after Vane's ship is destroyed, you are under fire from the nearby fort if you hadn't taken it already. During this mission, I disabled the fort so it no longer shot at me, but I couldn't overtake it because of being in combat.

Completing the mission, and subsequently being stranded on the island with Vane, I went through this section as normal. Then when I got back to Edwards island, I sailed out of port (after noodling around finding collectables) and then sunk to a nearby convoy.

When reloaded, it spawned me outside the fort Cabo De Cruz, the same fort I had disabled earlier.

Potentially could be a good way to skip some sailing but I am aware the speedrunners use fast travel quite frequently, but I thought of explaining what just happened. In no way am I myself a speedrunner I am just playing casually. Happy to answer any questions you may have!

This is the Steam version of Assassin's Creed IV Gold Edition on current patch.

Reddit link: https://www.reddit.com/r/speedrun/comments/16tonpb/potential_skip_in_assassins_creed_iv_black_flag/