The first two options to come to mind are Echo Lake Beach if you want civilization, or an easy hike up to Lonesome Lake to get away from the big crowds (and just deal with the smaller hut crowd).
You can also dip your toes in the Pemi alongside Lafayette Campground.
Or are you looking for ideas like swimming holes along brooks and rivers?
If you are headed into Lincoln, try The Ladies Bathtub. Its a deep hole in the Pemi. Beautiful water with public parking. Take Rt. 112 toward Loon, just before the mountain turn right on West Branch Road and follow around to the right. The parking is straight ahead and a path leads down to the river. I swim there after most hikes, also very dog friendly if this applies.