If you watch her closely, you can see how she is drinking. I would watch Bette lots she had an amazing technique were she didn't miss a drop even when she had a bowl of water she lapped it up.
When Bette was near the end, she began to lose large amounts of weight even though she had a healthy appetite and lots of other symptons which I haven't linked to Diabetes.
I would take her to the vet and get a urine sample done, sometimes the vet can get a sample by putting pressure on the bladder. This is the freshiest sample you can get. Failing that if you take a urine sample as close to the appointment as possible explain you think she is excessively drinking and can they test her urine for Glucose and ofcourse anything else the vet knows too look for. I must stress this is not an accurate diagnosis of Diabetes, as illness or stress can cause temporary elevated levels. However, when you follow this with a high glucose reading in the blood, it absolutely indicates diabetes. My vet did this by a prick behind the ears, all they need is one drop of blood.
This is a brief overview of diabetes in animals from what I gather it is the same as humans, dogs and cats.
http://www.caninediabetes.org/pdorg/diabetes_concepts.htm
It can be successfully treated , unfortunately I can't help there as I have no experience treating it. The above information, I found from researching and speaking to others who's pig had diabetes and it is my understanding of the correct diagnosis, thats not too say its right.
How many times are you having to refill the water bottle per day and what size is the bottle?