Assuming you haven't given up, let me try to respond to Zena briefly (as time doesn't permit me to do it at length before class today).

The short story is that I think Zena's response (like many attacks on externalism surrounding the issues we're discussing) trades on failing
to distinguish two claims:

        (a) if Otto is now thinking that water is wet, then he knows that he's thinking that water is wet  vs.
        (b) if Otto is now thinking that water is wet, then he knows that he's thinking that water is wet, rather than that twater is wet.

There is no reason to saddle externalism (or anyone else, for that matter) with the onus of showing how their view is compatible with
possibility (b).  Privileged self-knowledge does not entail being able to discriminate for any two twin contents whether one has a thought with
one rather than with the other.  In the particular case at hand, note that Otto is not even in a position to entertain the possibility that he
may be thinking that twater rather than water is wet - he simply doesn't even have the concept TWATER.

At crucial points Zena relies on the externalist's accepting that Otto "knows whether he's thinking that water is wet, rather than thinking
that twater is wet".  Of course, if Otto indeed knows he's thinking that water is wet, that entails he is thinking water is wet, which, in turn
(by externalism) entails that he's not thinking twater is wet.  But that doesn't mean Otto must know he's not thinking a TWATER thought if
he knows he's thinking a WATER-thought.

What is true of Otto is true of me.  As I go through the apriori reasoning, I recognize that the truth of externalism entails that
certain environmental conditions must obtain for me to have WATER-thoughts.  (Premise 1) I can then go on to affirm that I am having
a WATER-thought.   But I know, if  I accept the DBH line, that this doesn't mean I can tell ("apriori", in virtue of knowing what I'm
thinking) whether I am on earth or twin-earth.  So I can recognize that I will need to study my environment (aposteriori) to determine whether
there's water there or not.  I cannot avail myself to any shortcuts suggested by Boghossian, even as I accept externalism, and assure myself
I have self-knowledge.

So I still maintain that genuine acceptance of the DBH line affords blocking of Boghossian's argument.   (I here also want to refer back to
the last bit of my previous note, which isn't addressed by Zena, I don't think.)

(I realize the above may be too quick to help; I'd be happy to continue this later in some form or other.)

As for the DBH line, I don't quite see in what way it's "too good to be true"; I think it's both good and true … properly understood, of
course!  But defending that would take too long.  In any event, I think it is a mistake to treat twin-alternatives as on a full par with
skeptical alternatives in the case of the external world.  (Again, a good discussion of this, as well as of what self-knowledge entails in
this context is Falvey and Owens' "Externalism, Self-Knowledge, and Skepticism", Phil Rev. 1994.)

Another (after)thought, inspired by a question Ted asked on email a few days ago):

Consider Otto before 1750.  He would have known when he had a WATER thought, just as you or I do now.  Yet, of course, he wouldn't thereby know he had a WATER rather than TWATER thought, since those are distinguished ('de decto', as it were) only by reference to chemical properties of water of which people prior to 1750 were ignorant.  Still, Otto might have reasoned to the truth of externalism, I would imagine.  And that wouldn't give him apriori knowledge that water (i.e., something distinct from twater) exists in his environment.