Regarding the "north shore" recommendation, you'll find widely varied wading opportunities in the region north of Boston: sandy beaches, rocky shorelines, river mouthes, estuaries/back waters, etc. Use Googlemaps to look at the following towns: Beverly, Manchester-by-the-Sea, Magnolia, Gloucester, Rockport, Ipswich.
That said, I'm sure Cape Cod would offer great access, too.