To start this quest, talk to the Sea Hunter and choose Master Of The Sea. He will say the following:
Master of the Sea
I have to say, I'm surprised you've made it this far. You've proven yourself to be a very loyal member of the Sea Hunter's Guild. It's time for you to enter into the final rank.
Your task is simple. Retrieve a gemstone carried by Pirate Elite Captains. These Captains are quite tough and can be found on only the toughest of pirate islands. Look for islands with forts on them and you may find what you are looking for.
And don't ask me what the gemstone is for. I need it and that's all you need to know.