Hello,
Good idea to go diving in Marseille ! Unlike Frenchiegirl, I would not recommend going to Carry le Rouet, because, first the best dive sites are really close to Marseille and even rather on the east side (when Carry is at the west of Marseille), and also because I don't like much this diving center. I went there for several days and did not feel much welcomed. The boss was not friendly at all, and some of the dive guides seem to be fed up with their job !
In any case, I would recommend two centers in Marseille. I have not tried them myself but they have good reports from many divers, and both of them accept PADI divers.
They are Atoll Plongée and Archipel Plongée :
Centre de Plongée Atoll Club
Plongée Marseille - Accueil
I am sure they have someone who speak English....:blinking:
About the diving conditions, the visibility is usually good, or at least there are alwyays sites with good visibility. What could be troublesome is the Mistral (the local wind) which can make the sea very choppy and lowers the temperature of the air and the water.
Lastly, besides diving and watching rugby, be sure to taste a "Bouillabaisse" (a sort of fish soup) and go walking in the Calanque...
Hope you have a good time !
Malène