I've successfully launched out of the Winthrop boat ramp:https://www.google.com/maps/place/Winth ... 14a0f0a2e2
The sandy area off Morrissey Blvd south of UMass Boston:https://www.google.com/maps/place/584+W ... a08d49f356
You'll need look a tad bit south of the Yacht club, if you turn on Earth/Satellite layer you can see where the boats back down. Note: this location has a lot of sand and a TON of rocks. We stepped the mast and raised the sails on the trailer before backing down into the water, else you'll be scraping on the rocks the whole way if you try to drop the boat first and drag it in. I much prefer launching out of Winthrop.
Check out the Boston harbor islands if you launch out of either of these locations.
As far as the Cape, one of the sites I LOVE launching on inland is the Great Herring Pond in Bourne, MA:https://www.google.com/maps/place/Great ... 51d981f845
I just found this location last week and the lake is usually pretty sparse because most people that want to be in the water are 2-3 miles down the road at the beach. The water is beautifully warm and very calm. Also being so close to the shore you get some good wind.