As their name suggests, ghost ships are haunted ships set to the sea long after the last of their crew has passed on. As with a haunted house, a ghost ship will have absorbed the dead and is itself alive in a similar way to the spirits that sail it.
There are a large number of supposed ghost ships. The most well known one is that of the Flying Dutchman, captained by Davy Jones. The mere sighting of the ship was known as a death omen, as any sailor that spotted its sails would shortly after die in an accident. Davy Jones himself was known to be an extremely cruel captain in life, and in death would drown entire crews at once.
The sighting of a ghost ship in general was seen as an omen that nobody onboard would ever see land again, no matter the specific ship sighted.
A ghost ship is a ship that went down with crew still onboard, enough people for their ghosts to power the ship and allow it to continue sailing. Should a ship go down with no crew, or only one member, the ship will go to rest and the souls will move on. If the crew trapped onboard include the captain, however, the ship is more likely to rise and continue sailing regardless, often calling other lost souls at sea to join the crew. This can result in many lost souls joining one ship even if they had never been onboard.
The Flying Dutchman is a notable example of this, and is in fact well known for gathering souls lost at sea. These souls can eventually move on, but the longer they sail on a ghost ship, the more likely they become part of the ship itself, unable to ever leave unless the ship itself is sunk.
When a ship first rises, the crew will continue their last journey. If they can successfully complete the trip, the ship and its crew may be released to move on; in these cases, the ship returns to where it originally sunk. Ships that were making a single trip therefore rarely become ghost ships for long, as they have a final destination to allow their crew to disembark. Ships that didn’t have a specific goal, such as naval ships or marauding pirates, are the ones that become permanently haunted. These will continue their last mission, but often can never complete it and remain trapped, patrolling eternally.
The U.S. Navy considers lost submarines as still on patrol, passing messages and holiday greetings to all of these missing ships as though they are still completing their mission, and in some respects they still are. Their crew likely contains other lost souls from the wars they fought in, caught in the pull of the ship like a moth to a flame. Being a lost soul in the open ocean is a lonely existence, and souls will seek any company they can, even in who would once have been an enemy in life.
An established ghost ship may travel widely, depending on its final mission, and the sighting of one is indeed a bad omen, as bad weather and turbulent waters follow in their wake. Whether they cause them or intentionally sail within them is unknown, but wise sailors will take the warning they offer. The ships themselves are harmless; their crew will neither notice nor acknowledge the living; they are only halfway in the living world. Should one manage to step foot onboard a ghost ship, the crew will only be barely visible, moving to keep their home afloat without pause.
The ship itself may fight to keep you onboard, however, if you enter; they are kept moving by the collective energy of the dead, and a new soul is a way to continue moving. Any sailors who die in the wake of a ghost ship may find themselves onboard, and setting foot onboard is akin to entering a labyrinth. The rooms will warp and move, attempting to keep you inside long enough that you can join the crew.
While the ship and original crew will continue their original goal, new souls may pursue their own goal and therefore can be freed. As stated, though, the longer a soul is onboard, the more they mesh with the original ships goals, at which point only freeing the whole crew will release them. Some ships have been released by the end of the war they fought in, or the end of the dynasty they served. Others by the death of an enemy the captain had sworn to defeat, if the circumstances allow for it.
The ship itself can theoretically be exorcised, though no known attempts are documented, and the ship will fight to keep its crew. Sinking the ship itself could free the crew if the ship is thoroughly destroyed, however, the ship can repair itself over time, and may gather an entirely new crew if the first one has been released. Places like the Bermuda Triangle has many such ships sailing in it, which have been destroyed and been rebuilt by dozens of crews, the original crew long since moved on.
As with the banshee, they are an omen to be respected and heeded instead of dock the ship if possible.
If you wish to free the souls onboard, you are facing an uphill battle; the easiest way of doing this is determining the ships original goal and helping them accomplish it, without yourself dying in the storms that chase their wake. If this is accomplished, the ship should free its crew and sink back to the depths, but as you cannot ask the ship what it aims to do, this can be a trial to do.
You can also destroy the ship, but be aware: even if most souls onboard are freed, occasionally some souls remain attached to the ship, and nothing will free them. This is true of Davy Jones, who has captained the Flying Dutchman for so long he and the ship are the same, and no amount of destruction will keep him or the ship down for long.