I do not know of a bus connection. I took the train from St. Charles main station. It was inexpensive and It was only c. 3 stops and very easy. The Port is a bit far from the train station in Cassis but there is a little commuter bus that waits for folks getting off the train and takes them right to Cassis near the casino! We had a wonderful time- a short easy trip from Marseilles with the train but really a world apart!