The beach at Cape Tagarit is a wide sweep of Atlantic sand at the tip of the headland, completely undeveloped and almost certainly empty when you arrive. The water is clear, relatively calm in the sheltered bay side of the cape, and shallow enough for snorkelling close to shore. Turtles — mostly green turtles — use these waters regularly and can be seen from the beach or during a swim.
The Atlantic water here is cold by tropical standards — the Canary Current runs south along this coast — but the clarity and the marine life more than compensate. You will need to bring everything: the nearest shop or fuel is hours away. Camp on the beach, swim at dawn, watch the sun set over the Atlantic. This is as far from the tourist circuit as Mauritania gets, which is saying something.