When you book with a boat in the fleet, Continental offers a discount for flying into Portland. Avis is located at the Knox County airport in Owls Head.
Taxi service is available from there.
National's office in Rockland is very willing to drive you back to the schooner on Monday when you have returned your car. Avoid the city of Boston altogether by taking Interstate 495 North to Interstate 95 North. If you are traveling during mid-summer on a weekend take the less trafficked route, unless you have time to meander. Take Falmouth Exit 9 to Interstate 95 North. From Brunswick, continue on Route 1 North into Rockland.