We've found it worked well for us. It leaves you flexibility to move south when you're ready.

If you have a dog, you can get a one-way rental from Portland to Mount Desert Island (you'll have problem with intermediate point, I believe). Then you can use a loaner truck from a marina like Dysarts at SW Harbour to make the transfer.

Having the flexibility to move east and then south at yours and the weather's pace make a lot of sense.

Coming south, or going north (if you chose othersise), is easly done from Newcastle, to Marblehead, to PTown or Jamestown, and back through the Canal. With the weather, you can go from either Casco Bay or Boothbay to Newcastle (Portsmouth, NH) in a longish day. Then you have some greater choices moving south.

If your dog can last 7-8 hours on the boat, you should be okay except for the hop across to MDI.