Camping Virgen del Mar is in Santander itself - on the north coast of the peninsular. It's a large site although in the winter months most of it is closed off. It's not a brilliant site but because it's so close to the dockside it's nice to know that when you arrive in the dark, you'll be on site within half an hour. If you look at my Spanish blog from last winter you can read a full account of the site and see some pictures.
https://jondogoescaravanning.wordpress.com/another-winter-in-spain-2014-2015/
I've used the site, either on arrival or just before departure for the past three years. If you're familiar with GPS you'll find the site at N 43.47531°// W 3.87573° on the map.