The beach at Tham Saai (Sandy Cave) disappears at high tide, but is very nice when the water's low. (At high tide you can swim off the rocks, but watch out for cuts from the very sharp oyster shells.)
This beach is just to the seaward side of Wat Tham Yai Prik, and from time to time one may see the monks and nuns from the temple swimming, or sitting in meditation on the cliffs above. Be prepared for some heavy breathing when you climb 136 steps back up to Sripitsanu Bungalows, where you can get a cold drink and a snack.