The Leviathan is the name of your ship in the world '
The Caribbean'.
First, you will need
1700 White Crabs to upgrade your ship to its highest level which is
Level 9. Second, you can use either of two methods to do this which I will discuss below. Lastly, upgrading your ship to level 9 will unlock Jack Sparrows
Black Pearl for use in free roam.
Now the first method of leveling up is to destroy enemy ships. When you destroy a ship, you have a chance of White Crabs dropping from them but this method is not so reliable.
The absolute best method to do this is to head to an island called '
Horseshoe Island' which is to the far left of the map. On this island, you will find a big stack of barrels which you can destroy quickly with magic and will get 70+ crabs from. Head back to the ship and fast travel to
Port Royal to reset the area and fast travel back to the island. This method took me about
20 minutes to do.