in my opinion would be to pick up a mooring at Portland Yacht Services. It's a long but do-able walk to Whole Foods from there. Plus you can do laundry right at PYS, plus anything for the boat can be had footsteps away at Hamilton Marine or PYS's own very well-stocked parts department. Talk to Joanna and/or Phin Sprague, whp run the joint, and they may well lend you a car. They are very cruiser-friendly. Also, you can easily walk to downtown Portland for more exotic food options, restaurants, movies, etc.

The harbor there, admittedly, sucks, as it is terribly rolly with wakes, but it is the best option, I think, in terms of logistics.