The tiny River Alun enters the sea at Porthclais, which was once an important port for St David's and the surrounding area. The steep-sided inlet is protected by a harbour wall at its entrance, and has a small low-tide sandy beach just ouside. It's a popular venue for kayaking, scuba diving and sailing. East of the harbour, a rock slab provides good training for novice climbers. Facilities include a P&D car park, toilets and a small tea room.
A short distance to the east is the cove of Porth y Ffynnon. The shore consists of sandstone rocks and sand, with a narrow cave on the west side. Backed by high cliffs, it's only accessible on foot on a low spring tide, and even then, will involve much clambering over slippery rocks, and some wading.
To reach it, take a small path just east of the National Trust 'Porthclais' sign, heading around to the east and down to sea level. Proceed carefully and keep a safe distance from any sheer drops. Once at sea level and if the tide is low enough, you should be able to work your way around to the beach. It's best to visit just before the low tide, and once here note the position of the tideline relative to the rocks. When the incoming tide reaches that point again, it's time to leave.