Being an English concertina player, I' m not sure if I can help all that much, but...

I had been singing the Shoals of Herring before I took up the concertina some 30 years back. I began the transition by simply learning the tune (in C) and singing with it. The passage of time and the acquisition of a really good concertina has made me less reliant on playing the tune, aside from an interlude or two, and more comfortable with a chordal backup. This also makes it easier for the listener to comprehend the words.

Despite the notation in the EMPS Songbook, I would use a straight G rather than a G7. In the intro to "The Singing Island," Peggy Seeger advises avoiding the use of 7th chords as they are "too lush" for traditional music. Any complex or overly orchestrated accompaniments tend to detract from the song and do make it rather awkward for the listener.

Just some thoughts and opinions -- Tom