Crescent Island is on Little Sebago Lake. Little Sebago is the long skinny lake in this map. Little Sebago touches the towns of Gray, Raymond, and North Windham, Maine.
A closer view:
Crescent is the first little island to the upper left of the larger island. The Googles seem to want to call it Bass Island. And Bass Island is called Martin island. Oh well. So be it.
(5/2014) Success! Google now has the correct labels on their maps of the middle lake!