To get this you have to collect White Crabs that are found on the different islands in the caribbean. You'll need a total of 1700 of them to reach max level, which is level 9.
I found that the fastest method is going to the "Confinement Island". When you're there park your ship at the entrance and swim inside. There you'll find many barrels of crabs.
1) Destroy the barrels and collect the crabs
2) Lock on the ship with R1 and get yourself to the ship
3) Fast travel to the same island from the ship
4) Repeat
This will probably not take you more than 15-20 minutes depending on how much you've done before.