Not sure how you made your booking, but if you did it on one PNR through Expedia or some similar site, it often doesn't show all of the 9K flights BOS-MVY as routings, so a mis-connect may not be that bad. They usually have them nearly every hour until 9pm or so (check flycapeair.com). It's been a while, but the last time I flew 9K, they were very happy to switch my ticket to another flight later the same day for no fee.

Also note that DL and B6 are both running seasonal JFK-MVY this year.