While the answer from Primož is good, I thought I'd add a couple of places I had found:
Photo by CMaster, released under CC BY-SA
A 5 minute drive or probably 20 minute walk or so outside of Bovec, and a popular sight-seeing spot. There are two pools under the fall, one of significant size and depth making for a good swimming location. The water is however, really quite cold, and the position of the waterfall in a hollow means that is receives relatively little sun.
Just upstream of where the Lepana flows in to the Soca, is Kamp Soca. On the opposite bank to the campsite is a large parking area, and a gentle beach that leads in to two areas suitable for swimming, one of them even allowing some jumps from height as it is at the very end of the Velika Korita ("Big troughs of the Soca"). This spot gets very busy in summer however, with both passing traffic and residents of the campsite.