There are several methods that many people employ:
Anything of value that you don’t absolutely need should be left at home or locked out of sight in the car
Valuable items that you have to keep with you (phone, wallet, etc.) should be hidden and, if possible, locked away. Some beach blankets have hidden zippered pouches for this purpose, and certain coolers can be locked – again for this purpose.
Bring your items with you in a dry bag! These waterproof bags and wearable pouches are designed to allow you to carry your vulnerable items safely, even when you’re taking part in water sports.