As Platypus raises, this isn't a question that needs to be answered, though the mystery and its endless possibilities are definitely part of the fun.
Who knows, they could easily be ghosts in the typical sense that they're disembodied souls of the dead AND also residents of some other world. The way they mirror the sailors like disembodied shadows could support the ghost idea, and this wouldn't be the only time a weird author suggested Hell being another fold of dimensional reality. If I remember correctly, Hodgson even hinted at a sort of Heaven among the mind-bending cosmic folds of The House on the Borderland.
On a semi-related note, this isn't how I imagined them, but I enjoy Sidney Sime's illustration of the ghost pirates.
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Edited 1 time(s). Last edit at 4 Sep 19 | 09:43AM by Yluos.