Cassis is a cute little fishing village. We have been there a couple of times by rental car, but last May when the ship pulled into Marseilles, we decided to do it on our own by taking the train. Big mistake!
I downloaded all the train schedules ahead of time. We took a taxi from the pier to the train station, purchased round trip ticket from the machine in English, boarded the train, and arrived at the train station in Cassis. The station is about 2km up the hill from Cassis, and a bus was there to meet the train to take us into Cassis (for a couple of Euro). Easy Peasy up to this point.
Getting back was the issue. We noticed there were 7-8 taxis at the port, so we assumed no issue getting a taxi back to the train station. Wrong! When we were ready to leave, every one of the taxis there were already hired and just waiting for their party to return. There was not a taxi to be had in the entire town, so we were held hostage taking the bus back. Bus schedule was not frequent. When the bus came, we just caught the last train to get us to Marseilles in time. The train was a "milk run" so we got back late. There was a long queue to get a taxi to the pier. If it hadn't been for a ship's tour returning late, we would have missed the ship.
Rent a car or hire a taxi for the day, and don't rely and be held hostage by local transportation.