- the island-hopping Casco Bay Lines ferry, which operates on a limited schedule from Portland and takes quite some time (45-90 minutes), as its route has several stops;
- a water taxi from Portland (Portland Express Water Taxi or Island Water Taxi are two options), which is expensive; or
- the Chebeague Transportation Company's Chebeague Island Ferry departing from Yarmouth, which, if you ask me, is the only way to go.
A bonus: This is the ferry service most often used by islanders--who all seem to know each other--so you'll overhear authentic Maine accents and learn a bit about island life as you cross Casco Bay with residents who've been to Yarmouth to stock up on provisions.
It's just about a two-hour drive from Boston to the Chebeague Island Ferry parking lot, three hours and 45 minutes from Hartford, Connecticut.