The Harbormaster's pump-out boat services
boats in Weems Creek, Spa Creek and Back Creek, including boats in private slips or
marinas... so I'd guess they can also
service boats anchored in those two areas. It's not like there's tons of room for
anchoring in the latter two, though, except for on the outside of Spa Creek where it's a bit rougher (especially from wakes)... so temporarily rafting to pump out might take a bit of attention to timing.
Anyway, see here for info
Harbormaster's Office
-Chris