The main beach at Pozo Izquierdo (called El Arenal) isn't really a place for sunbathing and swimming as it tends to be windy and wavy and there are always windsurfers and kitesurfers zipping about in the shallow water. However, on calm days in the winter, it's perfectly safe to swim.
If you visit when there the wind is blowing, head to Playa del Ribanzo, a small, black-sand beach just to the north. It can lose its sand after winter storms but tends to be sandy in the summer. El Ribanzo is sheltered by a couple of stone jettys so is safe for swimming. Don't swim out beyond the jettys as you could easily be hit by a windsurfer.