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