The water along the Bay of Biscay can be dangerous and would-be swimmers need to be careful about the undertow. A Guardia at Zarautz was a diver working with the Search and Rescue unit which, she told me, should properly be called Search and Recover. Be really careful, check to see what colour flags are out, and if there are lifeguards, and pay attention to locals' advice.
I have walked along the del Norte three times and loved the beaches. Most of them I would not dream of going in over my waist. Here are a few exceptions: Zarautz, Playa de Santiago east of Zumaia, Deva, Oriñon, Tregandin (after El Brusco, if you have the nerve for it) which continues to Noja, Galizano, Los Tranquilos which continues to Somo, Torimbia, Santa Maria in Ribadesella, La Isla (very good for dawn swims), Cadavedo, Porcia, and a slew of wonderful small beaches west of Tapia de Casariego, finishing with a quick dip at the playa de Arnao before crossing the horrid bridge into Ribadeo.
There are other remarkable beaches, such Langre, Ballota, San Antolin, Aguilar, Tablizo and de Ballota east of Cadavedo, and Frexulfe, among many others, where I would cheerfully wade but go no further. I long ago heeded the advice of my swimming teacher who said that live chickens have a lot more fun than drowned eagles.