Ghost Ship

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In modern English, the term ghost ship has come to denote at least one of three separate (though occasionally overlapping) definitions, all of which involving, in one respect or other, unexplained circumstances. Historically, the term has been used to refer to reported sightings of apparitions over water that have appeared in the form of maritime sailing ships, often after having previously been known to have sunk, or to derelict vessels found floating with no crew. In fiction, ghost ships have often been vessels crewed by some manner of spectral or non-livingbeings.

In Supernatural
A Ghost ship can refer to either a ship that appears as an apparition after it has know to have sunk, or to a ship which continues to sail after its crew mysteriously vanishes.
Sam mentions that they are often a Death Omen.
The other ghost ships mentioned in this episode are:

• Espírito Santo -fictional ghost ship that appears in 3.06 Red Sky At Morning
• The Flying Dutchman
• SS Violet
• Griffon

In Legend
The main legend of ghost ships among mariners has been the Flying Dutchman, a captain condemned to eternally sail the seas. The legend has inspired several works.
According to some accounts, many ships responded to the desperate Morse code messages from the Dutch freighter Ourang Medan. The ship was found adrift off Indonesia with all of its crew dead. The boarding party found the entire crew "frozen, teeth baring, gaping at the sun." Before the ship could be towed to a home port, the ship exploded and sank. The reason for the deaths are still unexplained today. The original report of this incident cannot be located, and the entire episode is thought to be apocryphal.