Water needs to be available at all times. Through hanging a water bottle or placing a drinking tray the squirrel can consume the water. Both the bottle and tray need to be made from glass, so they can't gnaw on it. Be aware that glass can be dropped and break and that in winter the water can freeze and break the bottle if it’s filled all the way. For every residence I always use two or more water bottles. If one of them malfunctions, for example when the little sphere gets stuck, the squirrels can still drink water.
Of course it’s not essential to purchase these bottles. The squirrels can drink out of a drinking tray. However, in my experience these will get dirty in a day and then I would have to replace the tray with a clean one. Looking for my own ease and hygiene, I have drinking bottles hanging in the aviary’s instead of drinking trays.