I rent a house on the coast of Brooklin each year at the end of the summer, when the rates drop. I can tell you that the ocean water is freezing, even in August.
I have also rented one in nearby Surry, and I was told that in the late summer mackeral may be running.
I didn't care at either place that I didn't catch anything; it's beautiful up there in August and September. Sunny, dry and no more black flies. I was there for July 4th, and it was unusually hot and humid.
If you've never been in that area before, check out the town of Blue Hill, for the atmosphere, and drive all the way south to Stonington for the sunsets and the lobster fishing.
Let me know how you like it.