Because Maine is a coastal state with many islands, it has quite a few ferry services. These ferries, some of which carry both cars and people, travel between the mainland and islands, as well as north to Canada. Click on the above link to view the many ferry operators.
Issues pertaining to Maine roads and highways, such as I-95 redesignations, exit renumbering and road conditions.
Apart from hopping in a car or RV to visit Maine, there are mass transportation alternatives: trains and planes. The above link lists travel options.