A useful simplification and generalisation for Beginner to Lower Intermediate level students is that uncountable nouns cannot be counted. Sadly, this is not absolute; they are not exceptions to the rule. Many, if not most, "uncountables" can in fact be counted. Waters, fishes and two lagers all exist, and many "uncountable" plurals are actually quite common. Advanced learners should know this, which and when and why.