Charco de Isla Cangrejo is a natural swimming pool cut into the black volcanic edge south of the marina. When conditions are calm it can be a lovely place for a dip, with waves breaking outside and the pool itself sheltered by rock.
Use common sense here. Natural pools on Tenerife's west coast can become dangerous when swell is running, and wet rock is slippery. If the sea looks rough, enjoy the view and swim somewhere protected instead.