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/

It's an interesting idea and not one I had thought of exploiting in the past. It makes sense that this is how it would work, as disabling a fort sets your reload location to that fort without setting you on mission. So any quitwarp/cheatwarp/death would spawn you there. However I don't think it would be useful in any situation, the main reason being that the spot is a long way away from the fort, about 600m away and on the Jackdaw. If it took you to the actual fort and set you on foot then it could be very useful. However this does give me some ideas for things to test so I will try those and get back to you if I find anything.