I don't take my money belt to the beach. I carry enough cash for snacks or lunch while the money belt stays in my hotel room safe. If I don't have a safe, it goes in the hotel's safe. If that isn't available, I would use a locker at the train station. I wouldn't keep it locked in my rental car either.
The only way your money belt is safe at the beach is if you don't leave it alone. Either someone stays with it, or you put it in a water proof bag and have it with you all the time.
European beaches are just like American beaches (I grew up in San Diego), cars in parking lots get prowled and people who leave their valuables alone find them missing.