The Sara Long Bridge. If you're heading North on I-95 from Portsmouth, NH into Maine, it's the green bridge off to the right. It used to be a railroad bridge, and they'd discard broken or empty glassware as they passed over. Last time I dived there, I got a large bag so full of glass that I could hardly surface it. Consider bringing a lift bag.
Also, dive slack tide and watch for boats. You may want to go with a stationary flag outside of channel traffic, as the harbor patrol gets touchy about obstructing it. There's good access on the Maine side of the bridge.
I got a pre-prohibition Providence Brewing Company beer bottle, a 1906 J.F.Howard salad dressing bottle, and two cool antique 1 pint whiskey flasks among the "keepers". I left a few barnacle shells on each when I cleaned them, and they look good on the shelf.