Maine River and Sea Charters departs from Port Harbor Marine, also known as Spring Point Marina, located at the very end of Broadway in South Portland, Maine. Head South over the "Casco Bay Bridge" onto Broadway. Continue following Broadway until it terminates at Port Harbor Marine.
You may also want to use mapquest. Keep in mind that Port Harbor Marine is also known as Spring Point Marina. When using mapquest, the marina address is:
Spring Point Marina
1 Spring Point Dr
South Portland, ME