I cannot answer your question on where to purchase bottled water, but.....the ship's water is perfectly fine to drink. I have a very sensitive stomach and have never had any trouble with the ship's water. They process their own water onboard and it is just as good, if not better, than bottled water. And you don't have to worry about lugging it onboard with you.
Mary Lou Scanlon
NCL Pride of America April 24, 2010
NCL Epic February 12, 2011
RCCL Allure of the Seas - September 18, 2011
Celebrity Eclipse - February 11, 2012:
RCCL Navigator OTS - February 9, 2013