Boston Harbor Sailing has a memberships that let you take solings out on the Harbor. IMO far more interesting sailing than community boating on the river.

http://www.bostonharborsailing.com/bhsonar.html

After doing that for a year if you are still interested, I'd look at used boats and find a place to get a mooring. If you want to build one go for it, but that is likely to be a multi-year project so having a boat in the meanwhile might make sense. At some point I may be interested in setting something up where we do a group by of a large sailboat and time share it.