I would not even recommend getting a car until you leave Boston, it is not worth the expense to have while you're there, if you're not from Boston you DO NOT want to drive there. I'm from NYC and the Bostonians frighten me! It's very difficult to get around in a car, almost nothing is marked as it should be.

The hotel in Winthrop is the Inn at Crystal Cove. I used to live right across the street, it's a nice area, right near the beach, and very easy to get around.