I don't find this activity to be particularly suspicious(I'm in law enforcement) given some of the factors mentioned above -- not near a vulnerable infrastructure, not using scuba, not overly nervous, etc. What I would find peculiar is taking photographs/video of buildings, bridges, factories or surveilling thru binoculars. Things tourists usually aren't interested in. As far as reporting suspicious activity, you can call any law enforcement agency from the locals to the feds. Also, in MA you can contact the JATTF(Joint Anti-Terrorism Task Force) in Boston.
You are not paranoid, just vigilant. We were NOT vigiliant or watchful before 9-11 and that was part of the problem.
LobstaMan