Pereybere's beach scene is compact but very usable. Pereybere Public Beach is the obvious choice for swimming: sandy, sheltered, and close to food stalls. It is small, so arrive early if you want shade or a quieter patch of sand.
For more space, walk or drive north to Bain Boeuf Public Beach. It is less polished and not as convenient for a full beach day, but the views toward Coin de Mire are excellent and the pace is calmer.