You can rent cars at this airport, but this is not necessary. You will be able to Uber or Taxi to downtown Portland, where the hotels, restaurants, and ferry dock are within walking distance.
Portland, Maine in August is very busy, particularly the Old Port. We recommend booking lodging as soon as you know you are able to make it.
No, we are arranging a private ferry for all wedding guests to be transported from mainland Portland to Peaks Island. The ferry will depart from the Casco Bay Lines terminal on the Maine State Pier (56 Commercial Street).