Most towns of any size have municipal pools that cost a couple of euros. A lot of the time they insist on you wearing a swimming cap but they'll sell you one if you forget yours. My favourite pool is at Portomarin.
In other places it's possible to swim in the river. You can do this at Vega de Valcarce, and Sarria along with other places that don't come to mind at the moment.
Brr, we didn't even soak our feet in January, our big water treat was occasionally finding somewhere to stay with a hot bath. But we saw lots of places where we said "Wouldn't that be lovely to put your feet in in the summer". Are the rivers and streams clean enough for swimming along the woodland paths, my guess is that many of them would be in the more remote places.
Thanks for all your comments about bathing, and Wallyranger for the notes on the spots he knows about. I shall mark them in my guide book.
Dawn: Bra and pants will be fine, or less if nobody is around. But as a plump 61 year-old with a balance problem I cannot make a quick run for cover. Sil also suggests bikini type underwear to wear at all times. Wish I had my 20's figure back again. Am I being too optimistic thinking I will tone up all the flab? Maybe if I get to walk the 816 klms. from Somport to Santiago de Compostella. Here's hoping!
And Magnara: If ever you are walking near Tiermes on N.240 nr. Monasterio de Leire in the winter, if there has been a dry summer the Roman Baths appear again on the edge of the Pantano de Yesa, when the level of the dam drops. Theyre are hot sulphur springs and people bath there, sometimes nude, maybe surrounded by snow covered land! Amazing. Went there once but in company of my non-bathing, conservative Spanish family. Had to behave myself. Pity.
There are good swimming spots at San Justo de la Vega, just before Astorga, and in the river at Mansilla las Mulas. Pilgrim attire varied by nationality, apparently non-existent with the Germans, French-Canadians and Spanish, and more demurely by English-speakers. I saw a nice spot just by the bridge in Monreal (the Camino Aragones) but did not take advantage of it.
There is also good public pool near the albergue in the park on the western outskirts of Burgos-- when I was there in 2005, the attendant let me in for free.
The albergue in Ribadiso do Baixo is right beside a shallow river/creek. There's a nice concrete platform with steps down to the water where you can bask in the sun in your bikinis while you see pilgrims going downhill towards the albergue. One of the people I was walking with actually did that. She probably knew about the spot because the moment we arrived at the albergue she dropped her pants and went straight to the water in her bikini, shouting "Hola chicos!" to the guys on the road.