I think you can get your car delivered to the docks in Rockland. But there is regular taxi service in town that can take you to Owlshead(4 miles from Rockland Harbor) Heck I've rented a full size van truck one way out of Rockland. Speaking of which, if you plan ahead, you can often get some great flights(cheap!) out of Owlshead to urban areas to the south.

I think if you plan to head downeast from MDI it makes sense to base out of there. Maine driving is a challenge. Once you get above Portland, driving gets tougher and it starts to make sense to go by water.

You add 2 hours driving time to get to Acadia, that nearly doubles your Maine driving time, and don't confuse those miles with "highway"