Would adding 3 cups of sparking water make the watermelonade taste better or worse?

Sounds like a great idea to sub sparkling for still water!

i agree with monita-- if you're just substituting it in instead of regular water, then it's not going to dilute anything and will just add carbonation. sounds like a good plan!

It would depend on your personal taste- if you like "bubbles" then go with sparkling