As Eddie says, Santander City is not a small town - far from it - and appears a HUGE CITY in the summer. Wow, there are A LOT of people in that city and on the beach in the summer.
In fact, MOST of the small seaside villages of Cantábria are quickly converted into temporary cities, busting with (Spanish) tourism.
Two summers ago I was cruising the coast in Cantábria for a few days and was nearly impossible to find hotel availability. Finally did, in Comillas, but not even in the village. In fact, it was in a "backside village" of Comillas called "Trasvia" - and it's not even ON the beach but up in the hills in a nice "countryside" setting. Found a nice, (even) cheap, sizable new hotel called
Hotel Dunas de Oyambre (2-stars, no air conditioning but it was cool at night) back in the middle of nowhere, but a good, winding drive to the beach. I went NOT to the city beach but to another one called Playa Oyambre, part of el
Parque Natural de Oyambre which had very very few people on it - and no hotels around it.
But regarding your question, I'd say the vast majority of those signs you see in windows are put there by the owners and they aren't doing any additional advertising. Others have those signs in their windows all-year-round, inviting callers to ask if the place is available in the summer, charging incredibly high prices. Yet others contract management companies to rent their houses for them. A small few have websites and the owners do their own marketing.
I would imagine it to be EXTREMELY difficult to find a rental property available, ON THE BEACH, in Cantabria. But at least you're starting your search early so there's a chance.
Try the following:
http://www.cantabria.com/ http://es.rentalia.com/casas/cantabria/v38.htm