First of all, I'd like to express my personal thanks for having this resource online.
I'm having difficulty understanding the distinction between metaphysical possibility and logical possibility.
It is said that Kripke's example, "Water is H2O" is an example of a metaphysically necessary truth, but not a logically necessary one.
However, to me it seems that the extension of the terms "water" and "H2O" is the same, so the meaning of the statement is of the form A is A. (Isn't it with the meaning of a statement that logic is concerned, and not whichever semantically equivalent terms are used?) Isn't the statement that A is A logically necessary? A world where A is not A seems to be a violation of the law of identity.
I guess it's likely that I am wrong. What are my mistakes?