The de re/de dicto distinction is in play, here. It's not enough to say the word "water," we have to differentiate between, "Use of the word 'water' on Earth," and, "Use of the word 'water' on Twearth." Then:
- The word "water" as used on Earth refers only to H2O in all possible worlds.
- The word "water" as used on Twearth refers only to XYZ in all possible worlds.
- WaterEarthspeak is H2O in all possible worlds.
- WaterTwearthspeak is XYZ in all possible worlds.
At least, this is the kind of thing that Kripkeans are proposing. Imagining a situation where an Earthling ends up talking with Twearthlings and uses the word "water" does complicate matters, although supposing that possible worlds are closed off from each other might make the thought experiment into a non-starter.
Caveat: the above presupposes that the doctrine of possible analytic aposteriority is roughly correct. I myself am not educated enough to be confident that such a doctrine is correct/incorrect specifically, as things like Chalmers' semantic theory make me wonder if words like "water" are fully semantically indexed to/per possible worlds. Moreover, assuming that H2O has a specific geometrical/topological character, I am hard-pressed to imagine that a world with XYZ differing topologically from H2O would have its chemicals assemble into macroscopic objects indistinguishable from Earth-water. But so if Earth-water and Twearth-water are topologically equivalent, I don't know that we'd have grounds for saying that H2O ≠ XYZ in the first place.
To illustrate my own confusion:
You are on a trolley with your pet kittens Awe and Wonder. Like Luna and Artemis from Sailor Moon, your cats have symbols on their heads, a ? and a !. Suddenly, the trolley is derailed; your cats put their heads together, and a ?! forms and opens a portal to Twearth, upon which the trolley crashes with the passengers otherwise unharmed. While stuck on Twearth, you start to get thirsty, and go up to a pond. You're about to drink when Awe and Wonder spring into action again. "You can't drink that!" Awe says. "That's not H2O, that's XYZ!" Wonder explains. "Your body can't absorb it right!"
Meanwhile, there's no oxygen on Twearth, either, but you seem to be breathing just fine...
EDIT: iterated modality: in standard modal logic, iterating the operators is superfluous. For example, if something is possibly possible, it's just possible; if something is possibly necessary, it's just necessary; etc. However, perhaps the standard is wrong, or perhaps qualitative compounding is nontrivial. E.g.:
- It is epistemically possible that it is not metaphysically necessary that water is H2O. Or (slightly differently): it is epistemically possible that water is not H2O even if it is metaphysically necessary that water is H2O.
So, "The word 'water' on Earth refers only to H2O in all metaphysically possible worlds, but there are epistemically possible worlds where Earth's 'water' refers to XYZ," etc.