Other than some cash, perhaps a credit card, phone (if needed) and car keys, I would not take any "valuables" to the beach with me. You can buy water-proof bags of various sizes to take with you into the water. You can even buy a safe type thing to attach to your chair.

Personally, I take a small water-proof pouch that I wear around my neck if I don't have anyone staying on the beach. As far as a beach bag, it is usually in my sight or with me if I walk.

You can also ask your "neighbors" to keep an eye on anything you leave at your chairs when you swim or walk. Again, "valuables" really have no place at the beach.