Marseille is a big city with proper beaches inside its limits, which is unusual for a major European port. The water is clear but the beaches are not the white-sand pictures of the Côte d'Azur further east — most are pebbled or have small patches of imported sand.
The Plages du Prado, just south of the centre, are the obvious choice: a long crescent of small beaches created in the 1970s as a leisure area, with shaded grass behind, volleyball courts, lifeguards in summer, and easy bus or metro access. Crowded but reliable.
The Catalans beach, a short walk south of the Vieux-Port, is the closest sand to the centre — small but surrounded by cafés and within walking distance of dinner.
For something quieter, the rocky coves at the Pointe Rouge and Les Goudes on the far southern fringe of the city have clearer water and a more local feel; from Les Goudes a short walk leads into the first of the Calanques. Off Marseille altogether, the Frioul islands have a series of small coves where you can swim from the rocks.
The water is generally swimmable from May through October; July and August are warmest but most crowded. Beaches inside city limits sometimes close briefly after heavy rain washes pollution into the bay — check the city's water-quality flags.