Located on the west shore between the 2 mile markers and Harmon’s Beach.
From their website:
The beach, owned by Portland Water District is located on the west shore between the 2 mile markers and Harmon’s Beach. The Boating Club leases the beach & upholds rules set forth by the PWD. It is accessible only from the water. Only boats registered by legal residents of Standish or those belonging to members of Sebago Boating Club are permitted on the Sand Bar beach.