Cranberry juice. It has antioxidants!

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

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.

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

Odourless, tasteless, colorless, uncarbonated soda pop.

lacroix boiii

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

Ayran.

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

Cider

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.

OG Four Loko

Arnold palmers. Nothing beats em

milk i guess but my breath would stink

Oolong tea

I've been drinking brewed cacao (Cacao beans roasted, ground, and brewed like coffee) almost every day lately. So that.