Full Idea
My guess is that objectors to the deflationary account of the Twin Earth parable are confusing the essential properties of water with the question of what is essential for being water.
Gist of Idea
Keep distinct the essential properties of water, and application conditions for the word 'water'
Source
Frank Jackson (From Metaphysics to Ethics [1998], Ch.3)
Book Reference
Jackson,Frank: 'From Metaphysics to Ethics' [OUP 2000], p.79
A Reaction
That is, we must distinguish between the actual ontology of water's properties and the conditions under which we (in our society) apply the word 'water'. Interesting. The latter issue, though, might push us back towards internalism...