Full Idea
Once we have discovered that water (in the actual world) is H2O, nothing counts as a possible world in which water isn't H2O.
Gist of Idea
If water is H2O in the actual world, there is no possible world where it isn't H2O
Source
Hilary Putnam (Meaning and Reference [1973], p.159)
Book Reference
'Meaning and Reference', ed/tr. Moore,A.W. [OUP 1993], p.159
A Reaction
Presumably there could be a possible world in which water is a bit cloudy, so the fact that it is H2O is being judged as essential. Presumably the scientists in the possible world might discover that we are wrong about the chemistry of water?