I definitely want to see the Maine coast before I die. July average temps in Portland are 60/80. Tempting.
Go for it! The coast tends to be cooler than inland. It will be humid most days but usually bearable. Depending on how much time you have you can hit different spots along the coast. There are so many little towns with quaint shops and restaurants, you can go out sailing or whale watching and eat lobster to your heart's content. If you head up to Acadia you can get not only the coastal experience but also a taste of what Maine's interior looks like in Acadia National Park. Maine is our "get out of dodge" spot because it's close and has so many different types of areas to visit.