La Ciotat has a whole string of beaches starting right in the town, and heading out east, for 3 miles or more. These are sandy, access is very easy and restaurants are close by on the street. The best beach is probably Plage Lumiere. It has good amenities: bars and restaurants, fun things for kids, lifeguards in summer, and disabled access and facilities. It was even France's first ever non-smoking beach! As you continue out to the east from La Ciotat the beaches get rockier and less busy.
The most interesting part of the coast here is the Calanque de Figuerolles to the west of town. This is a relatively lush ravine with a small pebble beach at the end and a lovely enclosed bay dominated by a rock formation called the Bec de L'Aigle (Eagle's Beak). Just above the beach is a good restaurant, Chez Tania. You can use this to find the path to the beach as it is a little tucked away in a residential area. Not ideal for kids - no sand, deep water and no lifeguards - but a good find for adults.
Just off La Ciotat is an island called l'Ile Verte (Green Island), which can be reached by shuttle boat from the harbour in 10 minutes. This is a wooded island with three tiny beaches and a 2km walk around it. There is good snorkelling and diving round l'Ile Verte. Best avoided in July and August.
La Ciotat used to be a big naval shipyard, this has closed down, allowing La Ciotat to reveal its more delightful features - the litte port and old town, the beaches and watersports.