About water bottles. If you buy the drinks package it includes water, soft drinks and speciality coffee. You can get a bottle of water pretty much any time and take them back to your room. Hope this helps.
About the drinks package pricing - you might see a 30% off but you should be able to cancel and rebook if you do.
We had a set dining time (5.30pm) on our last cruise. The queues for the any time dining when we left were quite long. If you have a set time it might be worth keeping it. You can always try the buffet at anytime if you get peckish.
Enjoy your cruise