N.M.
I delivered at Alta Bates in Berkeley (crossing the bridge was not a problem), but I toured MGH as part of my labor and delivery classes and all the other expectant moms in that class delivered there. The rooms at MGH are HUGE, very calming, inviting, and comfortable. And from what I have heard from the nurses (the woman who taught my class was an LDN at MGH and a woman in my babysitting coop is a therapist there) and other moms who have delivered there, they are open to alternative birth arrangements. I don't know about a true water birth (not sure if they have the facilities to do it), but most hospitals and doctors these days (especially in the Bay Area) will deliver the baby in any position you want, including in the shower if that's where you feel most comfortable and are best able to manage your contractions.
Good Luck and Congratulations!