The train station in Cannes is a couple of blocks behind the see front around ten to fifteen minuters from the port. you should allow at least 1.5 hours from when you touch dry land in Cannes to travel to Monte Carlo. The 10 minutes walk to the Cannes train station , then there is likely to be a queue at the station ticket office.
On the current timetable, there is a train from Cannes to Monaco at 10.40 due in Monaco just after 11.30 but you will have to get off the ship pretty quickly to catch it comfortably.The next train at 11.47 doesn't get into Monaco until 1pm. If you make the earlier train,it would be reasonable to spend a couple of hours or so in Monte Carlo then take the 100 bus from there to the Nice old town area and explore there. The tram runs to Nice rail station for your journey back. Remember to build quite a lot of slack into your schedule as public transport is subject to frequent delays plus you could hit rush hour on your return.
Should you miss the earlier train, I would suggest leaving out Monte Carlo and to go only as far as Nice by rail. If you are running really late or the trains are on strike or you feel like a more relaxing day , i would consider taking the 200 bus to Antibes and dividing the available time between Antibes and Cannes instead.
The current rail timetable is here:
Thanks for your info...travelling in august 2009 & really want to see Prince Rainer's Car Collection...is this museum far from the rail station in Monte Carlo ?? Cheers Kathy Melb Australia.