I drink 4 liters or more a day (and I won't count the couple of cups of hot tea and the one Diet Coke with Splenda with lunch).
Then again, I've always been a very thirsty person, but I'm not diabetic.
Another non-diabetic reason could be salt intake - I have a harder time quenching that thirst when I'm eating lots of salt.
But I would still get tested for diabetes - everything I've ever read suggests that unquenchable thirst is a sign of it.