akgrown, living in Boston, Cape Ann is your best bet. It's fairly close, and offers some of the best diving in New England. There're lots of great shore diving sites, as well as numerous charter ops that will take you a little further out. The shore sites are pretty varied in terms of terrain, access, depth, and things to see. Folly Cove is really nice place - a very pretty wall. A bit of a pain at low tide (slippery boulders), but worth it. Cathedral is a wonderful site. Rocky ledges, fairly deep (can get into 80s), with lots of variety. Old Garden Beach is a nice easy sandy entry, with a nice shallow rock reef that's teeming with life. Plenty of others.
As far as dive shops, there're a few that are fairly close to you. I'm not personally familiar with the shops in the immediate Boston area, but there're a few. There's East Coast Divers in Brookline, United Divers in Somerville, North East Scuba in Chelmsford, and probably a few others I'm forgetting. All will be happy to rent you gear, I'm sure. Not sure as to which are people's favorites, I've never used the ones close to Boston, as I'm a bit further outside the city. Hope this helps!
-Roman.