There is a shop on the pier...but if you have time you can cross the main road by the port entrance and go up the hill in front of you...at the top on the left hand side there is a large shopping precinct and under it is a Carrefour hypermarket!!! We discovered it quite by accident last year! It's not obvious but look for the escalators in the precinct!! Don't think you would have a problem taking it on board...we always buy packs of bottled water ashore.
Thanks very much for that great info, it is very much appreciated.