I brush my tongue when brushing teeth; although it is always white and furry through the day. could this be due to not drinking enough water?

Possibly: Although it might be normal anatomy, or something else. Honestly, it's hard to say without actually clinically examining you. My recommendation would be to have your own dentist examine you and tell you for sure.

See a dentist: When you brush your tongue, do you get anything off the surface and does it stick to the brush? How long has this been going on? Have you been to a dentist to have it looked at? Any other white areas in your mouth that can be scraped off? Definitely see a dentist for evaluation.