Tea. There's a lot of room for variation anyway.

D₂O, and then make a living selling my piss to chemical manufacturers

e:

How Drinking Heavy Water Could Kill You

Algae and bacteria can live with 100% heavy water and no regular water. Plant and animal cells are more complex, so too much heavy water results in sickness or death. One key issue is that heavy water disrupts mitosis, the type of cell division used to repair injuries and grow new cells. The mitotic spindles of cells containing too much heavy water simply aren’t able to equally divide a cell to form two identical new ones.

But, you have to continuously drink and eat only heavy water for several days to see an effect. Replacing 20% of regular water in cells with heavy water is survivable for humans and other mammals (although not recommended). Swapping 25% of water with heavy water causes (sometimes irreversible) sterilization. Replacing 50% of water with heavy water is lethal. It’s not a pretty death, either. Heavy water poisoning resembles radiation poisoning or cytotoxic poisoning from chemotherapy.

https://sciencenotes.org/can-you-drink-heavy-water-is-it-safe/

fuck I changed my mind can I do green tea

Glass bottle coke

Horchata

My heart says unsweetened black iced tea, but after a bout of kidney stones I guess I'd have to go with an iced herbal tea. Maybe something minty.

Tequila, some ice

The only thing that is more water than water, Cors light. But now, really I'd prefer to drink my own piss, since coincidentally enough, Cors Light is also one of the few liquid that is more piss than piss.

And no "water with a twist of lemon/slice of cucumber" goofs. Water isn't allowed.

No sugar, no proteins?

Tap water has minerals.

Lemonsoda

Irn bru

P I S S

Probably hydrogen molecules fused with oxygen with another hydrogen