S is nice on your pretender because you can script it for returning when you are out site-searching.
Why don't you get rid of the B4 and take N4 instead? You get regen for your sacreds, regen on anyone you give a shroud to, and you get to find those forts. You won't find any blood sites underwater in any case.
You can wish for blood slaves to get into blood. With luck-3 you'll get Cthugul the Stargazer by the time you want to wish, and he's S5 with 3 misc slots (and a head, of course
).