If you have not a fully flexible ticket on a TGV - Train (usually the most expensive) then if you miss the train your ticket is gone and you cannot use it on any later train (and even if you have a fully flexible ticket you have to be very lucky that on any later train there are seats available as you only can use a TGV with booked seats...if you travel with an TER (Regional train) no problem, you can jump on any train at this day with an flexible ticket as there are no booked seats.
Also I heard that on the way from the pier to the railwaystation there is a draw-bridge, which might be open and therefore blocking your way. So even if you can leave the ship by 9am, it's not garantueed to be at the station by 10 am...