During development of the previous Assassin's Creed game, the developers thought it might be good fun to include ship-versus-ship combat as a sort of side activity. As it turned out, this light take on naval warfare proved to be one of the most popular sections of the game, and so the fourth entry in the series was built almost entirely around the concept.
Spread across Assassin's Creed 4's seas are a handful of particularly challenging Legendary ships which not only offer up a healthy chunk of change when defeated, but are also necessary for unlocking the Devil of the Caribbean Achievement or Trophy - depending on which swashbuckling brand of console you're rocking.
Each one of these ships typically has a special attack or a very particular vulnerability to your own attacks. Understanding these unique strengths and weaknesses is key to beating each opponent and cleaning up on each battle's cash reward.
While the rewards for destroying these vessels are excellent, make sure that you're very comfortable with the basics of naval warfare before heading out on the hunt, and ensure you've taken advantage of every upgrade available to you. As well as a nicely upgraded ship of your own, many of the tactics in these battles require quick positioning and a good understanding of your own firepower - something that you won't have if you've been skirting around ship-to-ship combat or investing your reales in other priorities.
La Dama Negra - Seranilla Region
To unlock the fight with the La Dama Negra, you'll first have to take the Serranilla fort, found in the southwestern area of the map. Be aware that this particular ship is clad in near-impenetrable armour, and you'll have to direct all of your firepower at its sole weak spot, the captain's cabin at the very rear of the ship.
Try to avoid passing the sides of the ship, as the broadside cannons of this vessel are particularly powerful. If you can maintain your rear position for as much of the fight as possible, you'll find it much easier to avoid attacks, while dishing out your own comfortably.
Maintain your position at the rear of the ship and ram whenever possible. Use your chain-shot and cannons as well, both to slow the ship down and to prevent it from gaining deadly range, all the while chipping away at the enemy's hull. The idea's pretty simple here: lock it down, then knock it down.
HMS Prince - Navassa Region
"Once the ship reaches around 25% of its health, it'll begin firing its rear cannons at you."
Once you've conquered the Navassa fort in the the southeastern quarter of the map, you'll be able to take on the HMS Prince, a ship that's heavily shrouded in fog. As well as a powerful side-cannon attack, the HMS Prince also has a devastating mortar attack - get caught adjacent to the ship, or to the front of it, and you'll have a real problem on your hands.
Move into position as quickly as possible at the start of the fight, and then use your chain-shot to slow it down, followed by your own mortar fire to chip away at its health. Just be aware that once the ship reaches around 25% of its health, it'll begin firing its rear cannons at you. If you got into the rear position right at the start of the fight, then you should have no trouble surviving through this damage until the battle ends in your favor.
El Impoluto - Dry Tortuga Region
Once you've defeated the Dry Tortuga Fort located in the northwest corner of the map, El Impoluto will appear. One of the toughest of the five legendary ships you'll face off against in Assassin's Creed 4, the crew's strategy is as simple as it is devastating: get up close at high speed and ram your ship into kindling.
The trick to beating this ship is to turn the tables and take the fight to your enemy. Stay close to the El Impoluto so that you're able to ram its hull. You can make use of your non-mortar weapons to dole out some additional damage, but the bulk of your attack will come from ramming. Just make sure you're bracing your ship whenever El Impoluto makes an attack in kind.
Don't be tempted to get away from the target and bring your mortar fire into play, as playing with range is a fast way to invite disaster in this fight. All you'll achieve is to make it easier for El Impoluto to line its special attack up against you, and then get a massive burst of speed in the tank for its devastating ram attack.
HMS Fearless & Royal Sovereign - Eleuthera Region
"As soon as you've destroyed the first ship, start moving away from the area as quickly as possible."
These two legendary ships are located in the northeastern section of the map, and they must be tackled together. It's critical that you avoid passing between the two ships, because otherwise you'll end up being caught between both vessels' side cannons. To take out your targets, simply use your mortar attack from range against a single ship, and keep moving around to maintain your distance from them.
Once you have one of these ships down to very low health, switch to the other target ship immediately and whittle its health down bit by bit. Once both ships are close to sinking, choose one to eliminate, and then prepare for an extremely strong counter-attack from the surviving vessel.
That attack will likely come in the form of a high-speed ram, so as soon as you've destroyed the first ship, start moving away from the area as quickly as possible, firing everything you've got at the remaining ship so that you can destroy it before it gains ground on you.
Note that you can use this fight as a source of infinite money in Assassin's Creed 4. By defeating one ship, you'll gain the reward of 10,000 Reales. Simply allow the remaining ship to destroy you and you can restart the mission, grinding out 10,000 Reales at a time. It's a cheap trick, but certainly an effective one.