Carbonated water, sparkling water, even "water with bubbles" could work. Water with gas, no.
Water with gas? - No way! For english speaking people means this term you'll serve them water with gasolin!
I just left the same answer in a strengthening exercise actually, and was wondering the same thing. I guess mineral water also (evidently from the name) contains minerals, but I would say nearly all sparkling water I come across is mineral.
I assume that it is because the word is in Accusative case after "п'ю". Вода is feminine, therefore the ending "а" changes to "у".
Similarly: "машина" -> "машину"