I don't know anyone specific, but perhaps can offer some leads, which should be fun even if you don't find a teacher.
Are you aware of the monthly Boston Area Sea Chantey and Maritime Sing at the USS Constitution Museum, normally free, and held at 2pm on third Sundays? I'm guessing you would enjoy that for its own sake (if you aren't going already) but you also might find someone there who does play concertina and would be willing to help. Next one is apparently 19 June 2016.
And of course there is the Mystic Seaport Sea Music Festival coming up 9 - 12 June 2106. Not really that far away, certainly enjoyable, and perhaps another good place to rub shoulders with possible leads in your quest for a teacher. Or at least to observe others performing, and learn that way.
found both of these events, and links to more information listed on www.bostonsongsessions.org
For a chantey session, most songs will be acapella and most participants prefer it that way (for a real shanty it is more authentic), but concertinas do show up occasionally. For stage performances, a singer with concertina accompaniment is far more likely.
As for dance tunes, the first real trick is to understand the feel of the dancing. There are some great contra-dances in the greater Boston area, and certainly worth going to a few, if it isn't something you are all ready familiar with. From my memories of 20 years ago, the Concord Scout House has frequent dances, and hosts some really great bands. The great dances with great bands is true of many other locations in the area, I only mention one to get you started.
Now, very few of the bands will have a concertina, but that doesn't really matter, just get the feel of the musical style. Listen to the interaction between the fiddle melody, and the harmony/rhythm of the piano. (a likely combination) Then get yourself some sheet music or recordings (depending which way you learn best) and try some of the tunes you like best. Probably easiest to start with tunes that are in C, G or D or the relative minors Am, Em, Bm but the majority of contra dance tunes will be in those keys anyway. I would start by just concentrating on the melody line (often the fiddle part), but add in some of that rhythmic harmony occasionally, once the melodies start to become comfortable. Adding harmonies may require re-adjustment of the fingering on some melody notes, to allow the desired bellows direction for harmony chords.
Lost of online sources for tunes. www.thesession.org is just one example.