No it's not one of the great lakes - it's a state park outside of Monticello, NY in the Catskills. The entire park is 1409 acres and includes a nice lake with a public swimming beach.
At the north end of the lake you can paddle up the feeder creek a while and observe beautiful scenery. Not the biggest lake, but it's very scenic and easy to get to.
Public Beach, hand launch, fishing dock, bathrooms, showers, snack stand, picnic tables, hiking trails.
$3 fee per person
Take Route 17 to exit 104. Take exit 104 onto Rte. 17B and follow for 10 miles. Make a left on Dr. Duggan Rd. The park is just a few miles on your right.
Website with more info: