Hi PietB,
My advice : choose another port for your holiday destination. Without any proper authorisation from the Port Authority, you wont be able to go ANYWHERE. I am lucky enough to have had authorisations in the past years since I work for maritime publications, but private individuals normally cannot get them.
If you still decide to go there, you will see that Marseille-Fos is a huge complex, very similar to Rotterdam/Europoort. Every dock is miles away from the other, so you usually end up the day with ridiculous mileage on your car (I did once 250 KM in one day) . A car is an ABSOLUTE MUST. And you also need a good petrol budget.
The traffic is another issue. Twenty years ago, the traditional port of Marseilles was very interesting, with a lot of conventional cargo vessels, old bulkers, some container vessels and typical Mediterranean roros and ferries and dodgy coasters. A very busy repair industry was also generating very interesting 'repair' traffic with large LNG vessels and super tankers coming to the port. All this is gone now. 90 per cent of the box traffic has gone to far away Fos (the container terminal lies 50 kms from the Marseilles docks !). Conventional cargo vessels have disappeared, most of the old roros have been scrapped and the ship repair business is NOTHING compared to what it was and mainly focuses on ferries and cruise vessels. This means that the traffic in the traditional docks of Marseilles has become very poor, except on week-ends where you may have up to 5-6 cruiseships in one day all arriving early in the morning and departing in the evening. And it is a big shame since you can take fabulous pictures from the port's 'Digue du Large' (the Marseilles breakwater).
Fos can be extremely frustrating since you keep shuttling between the various docks for only a few ships. And every time you have to go through the security checks to show yr authorisation from the port. A pain. The oil jetty which used to be accessible with a permit before the Sept 11 attacks is now completely closed even with a permit.
Ships going to the Fos Container Terminal may indeed be, in some cases, possible to photograph from the port jetty at Port St Louis du Rhone, but this depends on the size of the vessel, the orientation of the sun, the power of your lens. So not ideal really.
In short, I would recommend other ports. Have you tried Livorno ? A great place in the morning with free public access. Good and diversified shipping traffic. You can spend the afternoons visiting beautiful Tuscany !
Cheers
Charles