Full Idea
It is not metaphysically necessary that water is composed of H2O molecules, because the natural laws governing the chemical behaviour of hydrogen and oxygen atoms could have been significantly different, so they might not have composed that substance.
Gist of Idea
H2O isn't necessary, because different laws of nature might affect how O and H combine
Source
E.J. Lowe (What is the Source of Knowledge of Modal Truths? [2013], 6)
Book Reference
-: 'Mind' [-], p.23
A Reaction
I fear this may be incoherent, as science. See Bird on why salt must dissolve in water. There can't (I suspect) be a law which keeps O and H the same, and yet makes them combine differently.