A lot of my 'directs' are in Portland. Great city. August is peak, then July, early September. You would be advised to plan ahead for lodging if you're aiming for those months, anywhere in Maine.
We had a great time going in June when I was a teen, it's a bit early thus not super busy as New England area largely still in school and weather is sublime. Acadia NP gets busy. Upstate/inland Is a much different experience. I'd probably plan ahead on either salt water or fresh water.
Maine has a fairly regimented licensed guide regulations. My Dad and brother had a great sea duck hunting experience up the coast a bit.