We were just on the Wish and I was able to get plain seltzer/soda water at dinner no charge. I do not remember seeing seltzer at the self serve stations. Maybe you can get it at the bars.
You can get it in the dining room for free. I ask for soda water with lemon slices. I know sometimes they'll charge if they get it from a can (club soda), so I make sure they understand I just want the free stuff from the soda machine. From the Fantasy pool deck, I would usually get vitamin water, plain water, unsweetened iced tea, lemonade, etc vs soda.
GET UP TO A $1000 SHIPBOARD CREDIT AND AN EXCLUSIVE GIFT!
If you make your Disney Cruise Line reservation with Dreams Unlimited Travel you’ll receive these incredible shipboard credits to spend on your cruise!