Sorry I can't help you out about Sète, but strongly recommend your visiting the big Church on the top of the hill at Marseilles. It's called Notre Dame de la Garde, and was originally intended to extend Christ's protection to all setting sail and for them to give thanks for a safe return. It can be very windy on top, but the views over the city, its port and the offshore islands are incredible. There are also good loos and a coffee shop! The little train will take you there - it's a very steep walk.
Traffic is HORRENDOUS, but the main 'drag' leads off from the old port. There are loads of fashionable shops ... and even more people! Sanity, comfy shoes and a good map required! Have a lovely cruise.