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.