I don't know, maybe you're having a deep insight! Roughly speaking, the idea is that water in our universe is essentially H2O. The intuition is that if we went

I don't know, maybe you're having a deep insight! Roughly speaking, the idea is that water in our universe is essentially H2O. The intuition is that if we went to another universe where there was watery stuff that duck-typed as water but had underlying composition XYZ, it would not be water. Doesn't sound too controversial, but has interesting implications (e.g. it's an a posteriori necessity!). And there's also some interesting and powerful explications of "our universe" and "another universe" to do with the nature of counterfactuals. For an alternative take, see Dupré, John. (1995). The Disorder of Things. Harvard University Press. Introduces the delightful notion of "promiscuous realism"!