Their website, sadly, says no drinks of any kind are allowed to be brought on the ship. I was surprised they didn’t even allow one bottle of wine to be brought in! Might I suggest getting some individual packets of the diet iced teas to add to their mineral water. ALSO, I have worked around the water issue by taking an empty yeti cup on board in my checked baggage. I get ice buckets for my room several times a day. I let it melt and use that for drinking water. That combined with your packets might be your work around!
I am doing my first Costa cruise, on the Smeralda, leaving 1/26/24. I’ve cruised over 20 times but this is my first European!