Ferry service to Diamond Cove is available through Casco Bay Line. Fares and schedules are available at cascobaylines.com. The ferry terminal is located at 56 Commercial Street on the Maine State Pier.
However, our recommendation is to park at Bug Light Park South Portland, ME. Associated with Fore River Boat Facility. At this location, you can arrange for Fogg's Water Taxi service to pick you up and take you directly to Diamond Cove. Bug Light Park - GPS Madison Street, South Portland, Maine. Fogg's Water Taxi Service - 207-415-8493 (schedule this asap as it is a very popular service).