(You need to Login or Register is a neat webpage that allows you to scan a map of lakes in Waukesha County.  Since it is an old map, it doesn't show all the development but it does show lakes in proportion and location to each other.  Some look small enough to exclude motor traffic.

However, my guess is that if it is a lake, and it is deep enough float a boat, and it is in Waukesha County, it is covered with motorcraft.  I would suggest instead the mighty Rock River, located just outside the other (west) side of Oconomowoc.  Right now it doesn't flow fast, also you would have a high probability of never seeing another soul on that river.  Park at the wayside at Hwy 16 and the Rock...go up or downstream.