If the same functional and phenomenological role suffices for reference, then H2O composition is a contingent feature of our water, not an essential feature of water as such across all possible worlds.

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Contingent feature(as used in metaphysics and logic): A characteristic that something happens to have but could have been different—it's not necessary for the thing to be what it is. It's contingent that you were born in your particular city, but not contingent that you need oxygen to breathe.
Essential feature(as used in metaphysics): A characteristic that something must have in order to be that thing at all. Being composed of hydrogen and oxygen is arguably essential to water—without those elements, it wouldn't be water.
H2O(as an example of a scientific classification): The chemical formula for water, showing it's made of two hydrogen atoms bonded to one oxygen atom.
Phenomenological role(as used in philosophy of mind): How something feels or appears to our experience and consciousness. For water, this includes how it looks clear, feels wet, and tastes neutral.

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