The booking conditions for each cruise line vary but generally speaking:
Water levels can significantly affect an itinerary – High water levels may mean that the ship cannot get under bridges – Low levels may mean that the ship cannot get into ports. If you are already on board, changes to the itinerary will be made to give you the best possible experience through extended coach and/or hotel options.
If you are yet to travel, you will be given options prior to travel so you can decide whether you still want to travel.