Hi,
I can only recommend Camping Islares, 37 miles East of Santander towards Bilbao, near a town called Castro Urdiales, right on the coast, huge beach, all the facilities and a lovely little restaurant on site. There are two sites shown on the ACSI map closer to Santander , I would trust most of the ACSI sites, depending on when you are going you can save money offseason with their discount card, recouping the cost of the card in a few nights.