If you are going up against big ships (like frigates), position your ship near your target and then swim to the ship to take the crew out in close quarters combat. When you've done that, swim back to the Jackdaw and then hit each ship with one broadside. You immediately win the boarding sequence that follows, without having to go into heavy fighting.
Infinite Money (Exploit)
In Sequence 2: Memory 3, you'll encounter an optional objective requiring you to pickpocket all Templars. There are three of them in the area, which follows shortly after a checkpoint. You can pick their pockets, then chose the option to "Reload Last Checkpoint." When you do, you'll find that the money you pocketed remains in your possession. You can repeat the process as often as you like before continuing the mission, and you can also come back to the mission later in the game if you wish. It's possible to earn 35,000R after around 7 minutes of effort. This exploit may soon be patched, so you'll want to play using the game's initial version if that proves to be the case.
You can unlock a variety of costumes by satisfying certain conditions, including having saves and Uplay investments from previous games in the series that are available on the platform. The available costumes and the corresponding conditions required are listed below.
Altair Robes Requires data from first Assassin's Creed
Ezio Robes Requires data from Assassin's Creed 2
Connor Robes Requires data from Assassin's Creed 3
Altaïr Ibn-La’Ahad Outfit Requires Uplay data from 1 prior Assassin's Creed title
Connor Kenway Outfit Requires Uplay data from 3 prior Assassins Creed titles
Cat Outfit Purchase required pelts from any general store
Hunter Outfit Craft after purchasing the red howler monkey skin and the white jaguar pelt from any general store
Stealth Outfit Complete half of the the Assassin contracts
Mayan Outfit Open the secret door in Tulum after gathering the 16 Mayan stones by completing the Mayan Stelae missions
Templar Outfit Open the cage in the Great Inagua hideout using keys gained by completing the 16 Templar Hunt missions
Edward The Legend Outfit Play in single-player while wearing Edward's authentic pirate outfit (costs 20 Uplay points)
The Blades of Toledo Pack Play in single-player while wearing the Blades of Toledo (costs 30 Uplay points)
The Vengeful Daughter Pack Access the Lady Black's Vengeful Daughter costume in multiplayer (costs 40 Uplay points)
The IT Crowd
If you go to the map near the entrance to the modern-day Abstergo offices, you can overhear two co-workers talking about Chicago. If you wait a minute or two, they will begin talking about John from IT. The male worker comments that whenever he requests help, John replies smugly with "Have you tried turning it off and on again?" That appears to be a reference to the popular British comedy, "The IT Crowd."