If you always drink water and stay hydrated and yet your mouth is still dry you could have a condition that is known medically which is Xerostomia.
The medical condition Xerostomia is essentially a persistent lack of saliva production which means that you're not producing the needed saliva and need to drink more water to keep your mouth from becoming dry.
Doctors can usually give you medicine to help with the saliva production so your mouth is not constantly dry so if your mouth is always dry even after drinking water I would tell your doctor about it as it could be a medical condition.
Another reason your mouth could be dry is possible diabetes which can be treated by a doctor as well but it's likely just a problem of persistent lack of saliva production.