In reply to FreeloaderJoe:
If you want to go somewhere devoid of many other people I'd suggest looking at the Elan Valley. The Monks Trod path from Pont ar Elan heads up to llynau Cerrigllwydion Isaf and Uchaf. I've camped up there but have not swum in the llynau so no comment whatsoever on their swimmability. Other areas of mid Wales might offer possibilities, but I don't know anything about the swimming opportunities in the llynau north of the A44 and S.W of Nant y Moch res. It is good walking country though.
In the Beacons, your best bet for simming would be Llyn y Fan Fach and Llyn y Fan Fawr. The former will most likely be crawling though and the latter also gets busy, though less so overnight partly because there are fewer camping spots there. The upper Tawe between Llyn y Fan Fawr and the minor road between Glyntawe and Trecastle has some lovely dipping spots along the way as well.
