We brought some bottled water with us while travelling and for excursions. You can keep them in the refrigerator in your room (Mariner).
You can purchase them from within your room which are more costly (Evian??). You might be able to purchase elsewhere, but I cannot remember.
I also remember they were also selling larger, cold bottles of water as you were in line, exiting the ship for excursions. Have a great trip! Mariner is awesome!!!