If necessity here is epistemic rather than metaphysical, then 'water is essentially H2O' is a strong empirical generalization, not a claim about the intrinsic nature of water across all possible worlds.

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Empirical generalization(describing what 'water is H2O' would be if necessity is epistemic): A pattern or rule we've noticed from observing the real world repeatedly, rather than something we figured out through pure logic.
Epistemic: "Epistemic" relates to knowledge—how we know things, what counts as knowledge, and whether we can trust what we believe to be true. It comes from the Greek word for knowledge and is used to describe questions about the reliability and validity of our beliefs and understanding. For example, "epistemic humility" means acknowledging the limits of what you can actually know for certain.
Intrinsic nature(Used in Armstrong's definition of internal relations): The non-relational properties of an entity
Necessity (in philosophy)(describing what kind of claim is being made about supernatural things): Something that must be true or must exist; it couldn't possibly be otherwise (as opposed to something that just happens to be true but could have been different).

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