Do the rooms have air conditioning? Also how far is the hotel from the beach you can swim in?
To answer your question, the hotel is 5 star so a/c will not be an issue. I believe you could regulate it yourself, but not 100% sure.
As for the beach, there are various at a walking distance from the hotel, others which by city bus will be less tiring and there is another beach which you must take a ferry. However, if you will be going to Santander before May, the beach will be the least of your concerns as is might still be breezy and coldish.
Hope the answer helps and enjoy your stay in Europe.
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