Sims have a hidden Thirst motive, which is why you sometimes see them autonomously getting water/juice/milk, even when their Hunger is full. So them autonomously drinking is by design.
That being said, if they're "constantly" going for a glass of water, that sounds like a potential issue. Reading through this thread, though, I see lots of people saying their Sims rarely/never get glasses of water. Sounds pretty random, with different results for different people, yes?
Either way, I'll look into this in my own game, thank you much for the reports! :smile: