My son (20 months) is the same way with all flavors of juice. He used to drink juice all day & I changed diapers all day! I just severely cut down his juice intake to 1 sippy a day, with lunch. (Milk w/ breakfast & dinner) It was tough, but he gets water every time he asks for a drink between meals. I had to switch from clear sippys to solid colors so he can't see what is in them. He will take about 5 big swallows before he realizes it is not juice and then he hands it back to me and asks for juice. I just hand him back the water. When he is thirsty he will drink the whole sippy. When he is not he will ignore it and go on to something else without complaint. Most of the time he is not really thirsty. I think of it like this: he is asking for juice because of the taste,not because he is thirsty, and if he was asking for candy all day (hungry or not) I would not give it to him.
It took about 4 days for him to accept that it wasn't always going to be juice. So if you do try this method don't give up too quickly. It really really cuts down on the number of diapers you have to change a day!! (He still tries to get Daddy or Grandma to swap his water for juice.)
He was tested for diabetes at his 18 month appt & it was negative. But if he still drinks water all day I would definitely ask for a test.