First: I think you will find that Llanes is just across the bridge (i.e., in the Principality of Asturias, not the Autonomous Region of Cantabria).
There are three or four beautiful beaches between San Vicente de la Barquera and Comillas. We passed a few of them traveling between those two cities on 19 July: there were campgrounds full of French and German (and Spanish) tourists at each of them. But the beaches weren't crowded - there's plenty of room for everyone.
Unfortunately, when I was there (17-20 Jul) it was quite cool, windy and rainy. The 'red flag' prohibited swimming at playa Sardinero Wednesday and Thursday; but the surfers were 'having a ball.' Friday, when swimming was permitted, I was on a full day tour to San Vicente, Comillas and Santillana del Mar; so I had to defer my swimming until I reached la Rabida on la Costa de la Luz in Huelva Monday, 23 Jul.
Be sure to visit Antonio Gaudi's 'Caprichos' (an upscale Japanese restaurant since '91) in Comillas.
[ 07-30-2001: Message edited by: Eddie ]