Another New Englander here. I love Rhode Island, especially how the scenery is always changing and so many great waterfront spots. I grew up on the coast of Maine but lived in the Boston area for a long time too. My favorite place I lived in Mass was Hull, home of Nantasket Beach. Not a lot of seaglass there, but it's still a wonderful place to visit; very lively in the summer. People who I knew in the suburbs would say "man, Hull is so depressing in the winter" but I couldn't disagree more since I loved having a gigantic beach all to myself most of the year.
I've been passing this tip around to Hivers in hopes that someone will go there and post some pictures for us: Take the drive up the coast of Maine to Port Clyde, just south of Rockland, and take the ferry over to Monhegan Island. It is an old artist colony going back to the 1800s and once you get out there you'll see why, it's one of the most beautiful places ever. I live on the west coast now, but I'd love to make it to Monhegan again before I die.
To get back on topic to seaglass, out here on the west coast I visited "glass beach" in Fort Bragg CA; a whole beach completely covered in pebble sized seaglass. So cool!