 Even so, the first reading is not the usual interpretation of EXT. On a more standard construal, EXT holds that a water thought metaphysically implies some environmental condition E. Yet metaphysical implications, e.g., that“water is wet” implies “H2O is wet,” are not always apriori. (See Kripke 1972 and the entry on  Rigid Designators).  So the apriority of (1) is not guaranteed here, and the reductio seems in doubt.