From THE MERMAID (Child #289), the version in The Penguin Book of English Folk Songs, sung by Mr. James Herridge, Hampshire, 1906.

Last night last night when the moon shone bright
My mother she had sons five
But now she may look in the salt salt sea
And find but one alive

Does it mean that there remains just one son alive, or that there remain four, and one, the cabin boy, is the only one lost?

Thanks. R