Piémanson is really the end of the world (it should not be that safe in high season) about 12 km from civilsation.
On the other side of the river lies Port Louis (also with shops and services) (ferry costs were 4,5€) From there is a road, only 7 km from civisation, (Route Plage Napoléon) up to the beach of 'They de Roustan'. N 43 21 11.8, E 4 52 45.5
You can wild on the beach. Beautifull but lonely spot (not in summer though).
You'll see the tall ships for Port de Fos.