Full Idea
Your verdicts about whether the stuff on Twin Earth counts as water depends on whether you think of Twin Earth as a hypothesis about your actual environment or as a purely counterfactual possibility.
Gist of Idea
Your view of water depends on whether you start from the actual Earth or its counterfactual Twin
Source
Laura Schroeter (Two-Dimensional Semantics [2010], 2.2.3)
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.18
A Reaction
This is the 'two-dimensional semantics' approach to the Twin Earth problem, which splits meaning into two components. Whether you start from the actual world or from Twin Earth, you will rigidly designate the local wet stuff as 'water'.
Related Idea
Idea 14701 Array worlds along the horizontal, and contexts (world,person,time) along the vertical [Schroeter]