Robin,

it was the last entry in the paybook of those who died (and were buried) at sea, and would be deducted from the dead sailor's pay before it was handed over to his widow.

This was a practice recorded as being officially followed by the VOC (Dutch East Indies Co), but there is every reason to believe that the English East Indies Company also did it.

The song is about the womenfolk of a town seeing the merchantman on which their men sailed returning and wondering "will I be a widow tomorrow?" (i.e. when the ship docks and they learn the fate of their loved ones). Not at all uncommon - 30% crew losses on a single trip were regularly recorded.