Three Rows of Books
They already said that they can't physically change the past. The water in 2020 will probably be drank, used or get polluted by the time that person lives, so it's therefore impossible to send water back, according to the rules, or even if it's possible it's probably illegal. Like, imagin if people from the future just sucked up all the water from the past. I'm pretty sure they won't even exist in the future.
chichi32310:shes says water is precious so sell or trade her some for usefull items