See thread Penguin: The Mermaid for context. The question might better have been asked there.

The verse in question is number eight in Penguin, and didn't come from James Herridge (or Herage) at all; along with verses six and seven, it was added by the editors to bulk out his text. Verse eight, I am pretty sure, actually came from an early print form of the ballad, 'The Seamen's Distress', which appeared as the second part of 'The Glasgow Lasses Garland [composed of some excellent new songs]' (British Museum 11621.c.3(68)): presumed to have been printed in Newcastle, c.1765. Child prints the text as his example 289A.

The verse runs

Last night when the moon shin'd bright
My mother had sons five
But now she may look in the salt seas
And find but one alive.

There is nothing in the garland song to shed light on your question, so any interpretation will have to be a purely personal one. It is worth mentioning, I suppose, that an earlier verse in 289A states that, of a crew of 564, there were 95 survivors.