I live in Boston currently, and dive pretty much every weekend unless we get weathered out (including winter).
Within two hours of Boston, you can hit a ton of shore dive sites - Cape Ann has a bunch, Cape Cod has a good pond for practicing, Rhode Island has multiple shore dives as well. So you'll have PLENTY to choose from. However many of the sites are unremarkable, shallower-ish sites often with poor visibility and not always a ton of life. A lot of folks dive for lobster and scallops which is cool, just not my speed personally. Some of the more noteworthy shore dives have hazardous entries and aren't for everyone. However, there absolutely ARE some very enjoyable shore dives to be had - Folly Cove and Old Garden Beach come to mind. Ultimately depends on what you're in to. For me, the abundance of sites has had my shore diving experiences be more about skill work than sightseeing - with some notable exceptions.
Boat dives are another story, however. In my opinion this is where New England diving shines. Shipwrecks, wall dives, etc. Some pretty rad sites. Plenty of charters operate within an hour or so of Boston. Boston Scuba even operates a dive boat right from the city proper! Very cool.
As mentioned, waters get COLD around here. I recommend exploring the possibility of a drysuit. There are divers who dive year round, and various dive clubs you can join to meet buddies and learn about the local spots.
Good luck and enjoy!