Singer complains of having married a sickly wife. After he introduces her to cod liver oil, she goes wild for it, demanding it all the time. He warns young men to avoid sickly women, or they'll "end up a-swimmin' in cod liver oil!"
Cod liver oil, which contains Vitamin D in quantity, was touted as a cure-all in the 19th and early 20th centuries -- indeed, it was still being given to gagging children when I was growing up in the 1950s. - PJS
The theme is not very different from that of "The Dumb Wife" [Laws Q5], in which a man, to his eventual sorrow, goes to a doctor -- sometimes named John -- to cure his otherwise perfect wife of her inability to speak.
Newfoundland authorship attribution is not always to be treated as gospel. Blondahl notes "there are several popular versions of Cod-Liver Oil, the original to be credited to John Burke." Burke (1851-1930) is a very well known author of songs in Newfoundland. In Blondahl's version the potion comes from "dear Doctor John" and not Doctor de [or D.E.] Jongh. If Burke is indeed the author his work made its way to Ireland. - BS