My dog would be a bad comparison. I have to watch out for dehydration with him. He forgets to drink. And he doesn't like chlorinated water that comes from most municipal water systems. Count yourself lucky at least in that regard!
Have you ever tried to let her drink her fill? Perhaps she's simply hording the water when she can, because it's not always available. She's still very young and she may think that water isn't always available - it's hard to tell from your post if you refill after she finishes the bowl.
Boxers also "seem" to drink more because they loose half of it flinging it around the room or as it drips from their flews when they come to wipe their lips on your pants leg or couch to dry off. LOL
So I'm not sure what to tell you.
I had one boxer who would drink and drink and drink until he'd had enough. Far more than the two girls. Then he was done. There was never anything wrong with him.
But:
A dog who drinks too much can have several different medical issues. The only way you will know is by having your vet do some tests.
Good luck!