August is the most crowded around here, but compared to the places you're coming from, things will be much quieter. Places like Camden can be very busy in August, but a mile away, Rockport Harbor will be quiet. Most of the Maine coast will be somewhere between the two for crowds.
As soon as the kids go back to school, it's like someone turned down the spigot on tourists, not all the way, but almost. Funny though, it's very similar in those same places around Cape Cod. We felt like we had Cuttyhunk and Marthas Vineyard to ourselves last September.
Weatherwise, there's little difference in the temperatures along the coast between August and September but there is less fog in September(Sep and Oct. are my favorite months for sailing here). August is a better bet than July for weather in my experience. Storms(like Hurricanes) are much less of a risk here on the coast of Maine with our cold water than the Cape.
Sounds like a great trip Craig.