A Ghost ship can refer to either a ship that appears as an apparition after it is known to have sunk, or to a ship which continues to sail after its crew mysteriously vanishes.Wikipedia
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