Visit the captain on the ship in Pirate's Cove. A solid silver treasure chest will be randomly placed somewhere in Pirate's Cove. Find it and return to the captain, he will take the chest from you, then go 2 north, down all south to gain the mark.
After receiving the mark, you may take 1 item from the chest. If you return the item to its owner you will gain +100 points to your reputation. Though with the change to reputation in early 2019, this no longer seems to be the case.
Note: As of January 2010 this mark is enabled for male and female.
For helping out your pirate brethren, the Powers granted thee a Mark. You have been granted The Mark of Brotherhood!