The lake can be divided into four distinct basins. Big Basin is located in the northwest corner of the lake and is approximately 5 ½ miles east-west and 6 miles north-south. The 316 foot (97 m) lake depth is found in this basin. Seismic reflection profiling done by the author as part of his Master's thesis (in progress) at the University of Maine shows that the bedrock surface is greater than 360 feet below the lake surface in the central part of the basin.
Here's the depth map.
So nobody seems to have made any connection with it being an impact crater. But I think I'll keep imagining it that way anyway.