The family jewels will recover. The bigger issue to be aware of is riptides. Some beaches on the north coast are particularly notorious:
Nearly half of fatalities on beaches and in pools this year have occurred in summer period
english.elpais.com
As for river and lake swimming, I have seen official bathing areas in some rivers. I assumed that European regulations would mean that the water at such official spots is monitored, but I don't know for sure. I have swum at Rionegro del Puente on the Camino Sanabres. Very nice.
Apparently, one of the 20 most beautiful spots in Spain is "Pozas de Mougás (Pontevedra)," at Mougas in the county of Oia, on the coastal Camino Portugues. It doesn't look too far from the camino ... mind you, since the camino goes along the coast, you have no shortage of swimming opportunities in the sea.
Find local businesses, view maps and get driving directions in Google Maps.
goo.gl
Video here:
If you're serious about maximizing your opportunities to swim, perhaps you should consider buying this guide to wild swimming spots throughout Spain:
www.wildthingspublishing.com
The website for the hardback edition includes a preview, where you can see a map of the swimming spots in the general area that the
Camino Frances passes through. There are also a few on the VDLP near Caceres / Plasencia. Some (most?) of these will require a significant detour from the camino, but that may be worthwhile if your passion is wild swimming:
www.wildthingspublishing.com