Every month or so, the Oceanmasters announce an event incentivizing a particular facet of game play by increasing the awards associated with that activity. We're in the middle of just such an event now. It's called "Brave the Graveyard," and it'll be going on until noon PDT on Wednesday, March 19.
Until the end of the event, it's trips to the Haunted Seas that will get you more ship graveyards to plunder, increased treasure if you port safely after your adventure, and a higher chance of injury if you sink.
What, you ask, are the Haunted Seas? They are the realm of Barnabas the Pale, a fierce yet deceased Brigand King. They function as a type of sea monster hunt. That is, for a ship to enter those dangerous waters, there must be a special chart on the navigation table, and all pirates aboard must have a Bravery Badge or a subscription (depending on your Ocean) in order to participate. The Haunted Seas exist temporarily upon a league point not accessible by normal means--hence the special chart. Once you enter, you will find yourself on a multiship battle board teeming with spectral ships which may pass straight through solid matter as though they were... wait for it... ghosts. Opportunities to haul treasure from sunken ships will arise from time to time, affording a chance at extra treasure and special trinkets stowed away in chests of varying sizes.
The main attraction upon the Haunted Seas is the ship graveyard. Sail into that sucker, and every pirate aboard will have to face his or her ghostly doppelganger in an epic swordfight. Don't expect your opponents to play fair, matey. Their sprinkles and strikes include a purple block which will harden into an unbreakable obstacle if left in play too long. Should your crew win the swordfight, you'll have a chance to haul up a huge cache of treasure before the ghostly ships close in once more.
If you've never been, now would be the perfect time to try it out. Watch the Notice Board's "Voyages" tab for job offers under the heading of "Sea Monster Hunting." In the "Exploring" column, look for a chart emblazoned with a red skull.