I have a half-remembered song from my childhood running through my head. It begins:

Oh, once I heard a mermaid sing
As I sailed this sea, this deep blue sea
Once I heard a mermaid sing
As I sailed this briny ocean

It's sung by a cabin boy, resisting temptation. I remember bits and pieces of other verses, but never knew the title or its origins. It might be from the 1970's, or it might be centuries old, or I might've made it up without realizing it.

Do any of you recognize it, or have suggestions as to where I might find it? I've tried Google, searching this site, rec.music.folk, and The Gunroom.