Polar seltzer. That's sparkling water, is that ok?

Man, it had want to be a HUGE drink! What happens if I run out?!

Cranberry juice. It has antioxidants!

I'd say Coke Zero, but technically this is just a "water with some chemicals in it" goof, and therefore not allowed.

Mtn Dew Baja Blast. And it's not even a contest.

Are we assuming the drink magically has no calories or other negative effects?

I'd choose white wine if it doesn't get me drunk (and as long as it still tastes good and not shitty like alcohol free wine. And also as long as there's no social repercussions from drinking wine all day.)

If the drink keeps all it's normal properties then I'd choose green tea.

Decaf tea. Though that's just water with a sprinkle of leaves. Not so different from a twist of lemon or a slice of cucumber.

Some herbal tea. Mint would probably be best. It's what I drink most of the time anyway.

It has to be refreshing, not too sweet. Because you'd need to drink a lot every day to stay alive, off the top of my head rice water sounds like an ideal drink, I love the flavor and it has less sugar than horchata, you don't want to have to stuff yourself with sugar every day when you're thirsty.
Another possibility is hibiscus water. Again - if I could regulate the sugar levels it would be a way to go.

Unsweat tea

lacroix boiii

Odourless, tasteless, colorless, uncarbonated soda pop.

Ayran.

OG Four Loko

Cider

Cactus cooler.

It's an orange pineapple carbonated beverage that if you can find is such a happy experience.

Coca Cola. I love it, but I try not to drink it too much. A nice iced can of Coke every now and then.

Barley Tea. Sure it has water, but what drink doesn't? (Also I feel like my days would be heavily numbered if I don't drink any water for the rest of my life.)

H1O mixed with some Hydrogen. Check mate.