public transportation from cruise port in Marsailles France

I believe when you refer to the 'old town' you mean the 'old port' or Vieux Port, which is the center of most everything of interest to the normal tourist.

I've seen cruise ships splattered all up and down the port complex.

If you're lucky enough to dock at the Port Autonome, the Joilette metro station is right outside, or you can walk into the old part of the city in about ten minutes (it's about half a mile).

If you dock at the far north end of the complex, I would suspect that either there's a shuttle or you need to take a taxi (the distance is close to five miles, probably). I've never noticed what looked like regular public transportation out that way.

The passanger train station is about a mile from the Old Port and pretty close to the same distance from the Port Autonome. The only trains I've ever noticed in the vicinity of the port complex are freight trains.