No the temps are pretty much surface temperatures.
No, they change an awful lot, especially on a shallow lake like Erie. The water temperatures off of Cleveland are measured at a location about 40' below the surface, and those temps surge into the 70s over the summer and dip into the lower-mid 30s in winter. Someone can correct me if I'm wrong with the depth of the Cleveland water temperature reading.
Here is the data:
BTW. Lake Erie only gets about 80' deep. It is a very shallow lake for its size.
EDIT: Yes I know this post in not accurate. I meant to say. Most of Lake Erie is around 80' deep. I know there is a area on the eastern side that is 64 meters (~210').