There aren't too many options for freshwater diving around here. That's not to say that you can't try jumping into any body of water you find, but most of them are likely to be pretty awful, and a few could get you arrested. Two good freshwater dives in the region are Round Valley Reservoir and the Delaware River Water Gap. Some diving is also done in Lake Hopatcong, although access is a problem.
Note: In 2020 Round Valley began a period of maintenance which is expected to last several years. During this time, the water volume is reduced by a third. My completely offhand guess is that this will reduce the water level by around ten feet. In the shallow areas where diving is practical, this will significantly alter the shoreline.