There is more than one story involved.  (Pretty) Nancy of Yarmouth, also called The Sea Storm (Roud 407) is about, well, a storm at sea.  Peter Bellamy sang it with the Young Tradition, but the text had been re-written (by him, I think?) retaining the traditional first verse and then going on to tell a quite different story.  See:

Pretty Nancy of Yarmouth  DT file; text and tune; taken from a book I've never seen, but transcribed from Bellamy's singing, I suspect, rather than from any traditional source.  Could be wrong, mind.

For Sea Storm texts, see these at the  Bodleian Library Broadside Collection.:

Nancy of Yarmouth  Printed between 1819 and 1844 by J. Pitts of Seven Dials, London.
Pretty Nancy of Yarmouth  Printed by J.V. Quick of Clerkenwell, London.

Roy Palmer (Bushes and Briars, 1983/99) considers the original to have been Pretty Nancy of London, in Leadenhall Street, which appeared in Lord Anson's Garland around 1757.

Then there's another story, in which Nancy's boyfriend is sent off to sea by her disapproving father, and (usually) murdered en route by a hired assassin.  His ghost reveals the truth to her, and she dies as well.  See:

Nancy of Yarmouth  (Jemmy and Nancy; The Barbadoes Lady) [Laws M38]  -The entry at the  Traditional Ballad Index.

Jimmy and Nancy  DT version from Nova Scotia.

Jamie & Nancy's garland: or the Yarmouth tragedy  Printed between 1821 and 1827 by T. Bloomer of Birmingham.  (Broadside at the Bodleian).

There is also what seems to be an intermediate story, in which Nancy laments for her lover at sea, but he just gets drowned by accident and we don't get a ghost.

At the Bodleian:

Nancy's complaint for her sweetheart Jemmy  Printed between 1790 and 1840 by J. Jennings of London.
Nancy's lament for her sailor lad  Printed by R. Evans of Chester.

There's even one with a happy ending:

The Jolly Young Sailor  -Version in the DT.

There's more, of course, but that should be enough to be going on with.  Is any of this anything to do with what you were looking for?

Malcolm