Casco Bay Lines runs scheduled ferries from the Old Port to the inhabited islands of Casco Bay. The three-hour mailboat run is the easiest and best way to see the bay: you pass close to Peaks, Little Diamond, Great Diamond, Long, Chebeague, and Cliff Islands, with twenty-minute stops at most of them.
If you have a full day, get off at Peaks Island (fifteen minutes from the terminal) and rent a bicycle for the perimeter loop, or Chebeague for a quieter day of walking. The boats run year-round; in winter you essentially have the bay to yourself.