Look on
DCL website under Stateroom gifts to find the prices. I don't believe that is something you need to go through your travel agent for -- anyone can purchase it for you as a gift to be delivered in your room, and you pay for it separate from your reservation.
I know everyone has different tastes, but I really can't taste any difference in the water I get at the soda machines (they have a separate spigot for the water, next to the sodas and lemonade) than the water at home. I just bring a tervis and fill up using a DCL cup ( you are not supposed to put your own containers under the spigot) or just grab a cup of ice water when I am thirsty between meals. I also order ice water every time I get room service, then keep the glass handy to drink from.
Finally, unless the rules have changed (which is VERY possible) you can bring your own water on board.