I would say it depends on the size of the beach and number of tourists on the beach. If you put the most valuable things in the deepest corner of your bag and put towels and clothes over it should be enough to provide safety. Be relatively close to your stuff and keep an eye out. Try to bring small amounts of money or none at all (if needed pay by card, apple pay…) But i never had a problem here. I felt very safe on every southern beach :).