Here is the Giant Squid Easter Egg I found in Assassin’s Creed IV.

The Squid is located in Antocha Wreck.
The Coordinates are (630,660).

You need to get Inside the Shipwreck and swim up the Stairs. You should be seeing some Tentacles that are escaping to a window (if you don’t then go back up again and try another time).

Swim to the window and wait to see the clip shown.

Unfortunately you can’t interact with the squid 🙁

I would’ve loved a Kenway Tentacle Rape Fight 😀