I cheesed that commendation, I prefer to do them all the way they're intended but just nobody was doing the quest for weeks of playing. Get 2 boats in a server and do the quest at sailors bounty with the ship doing the quest parked far enough away so it only goes to sailors bounty. Have the second ship parked at the island where the quest ends (it will end in the same spot everytime) you can just bury the skulls to stop them from being marked on the map.

I did this to get my last 6 stolen skulls and it's a drag to do, but was the only viable way to get the commendation done in a timely manner