I agree, I doubt sailors were playing english-system concertinas, which were considered "sophisticated" instruments from the time of their invention (see Atlas' book on Wheatstone). But there's plenty of evidence that the Anglo-German system was around, especially as you say, in the South Pacific. One of the journals I read talked about a whaling captain purchasing a concertina for one of his sailors at Bay of Islands, NZ. circa 1860.
My dissertation has to do with musical interactions between Euro-American sailors and Pacific Islanders in the 18th and mostly 19th centuries. I'm actually giving a paper on the topic at the Mystic Seaport sea music symposium in June. I hope to be done with the project about a year from now, and someday have it published. Someday . . .