While Brewster has only bay beaches, Harwich has only ocean beaches along Nantucket Sound (aside from the freshwater beaches). To park at the beaches, you will need a beach sticker. Here are the beaches of Harwich.
There are also a few Harwich beaches which are public, but have walking access only (and there are no lifeguards or restrooms). These are Belmont Road Beach, Brooks Road Beach, Merkel Road Beach, and Neel Road Beach.