"I drink water."
Translation:Tôi uống nước.
Because water is an uncountable noun. You can't count water so there's no use of a classifier for it.
like dd721411 said, uncountable nouns don't have classifiers. to be able to count them, we would use units of measurement: hai ly nước (two glasses of water), một chai nước (a bottle of water), năm lít nước (five litres of water), etc.
note that they are units of measurement, and not instruments of measurement, in which case they would require a classifier: "hai ly nước" vs "hai cái ly"