T.W.
I keep a veggie tray and water sitting on the kitchen table like I am having a get together every day from about 10:30 to 11:30 then again from 1:30 to 2:30. The kids keep coming in and out and take a drink and a veggie as they want.
I have noticed that a lot of times they drink when they need it but I do remind them a lot. I would just try to have water out for them to notice and remind them every so often.
Good luck!