Key points of World map - Legendary Ships - Assassin's Creed IV: Black Flag
On the Caribbean waters, there are four (five, in practice) legendary ships. They are hidden in the four corners of the map, always in the same locations. Each of these has the 75th level, a lot of health points and formidable weaponry. The reward for defeating them is 20 thousand regals.
The legendary ships cannot be boarded. After you drain their health points to zero, they sink. The only thing they leave behind is the money.
You need to be properly prepared for fighting the legendary ships. It is compulsory that you equip the jackdaw with legendary hull and cannons. Depending on the vessel, it is also going to be important that you have legendary mortars, fire barrels, chase cannons cannonballs and swivel gun. Therefore, the best moment to attack them is when you already have all the legendary design plans and ship upgrades.