One of my girls drinks at least twice as much as the other, but both are drinking a healthy amount for them and neither has any health issues. We have had a guinea pig with diabetes, but it is not common; her diabetes was controlled by medication (not all cases of diabetes even need medication, but hers did), and she led a normal life. My point? Try not to fret if your guinea pigs are drinking different amounts of water, as it is not necessarily an indication of an issue. It's not a death sentence even if a piggy does have diabetes (and I'm not saying yours does); it can be as simple as a slight change in diet.