FWIW, on Selene, as with others who have responded, the sink and shower connect to a "grey water" holding tank just abaft of the mast. The pump is next to the mast, starboard side (access by removing the plastic bin under the settee).
The switch is a funky pullout switch on the forward bulkhead in the head area, as rnebergall stated.
As Chance said (nice idea about the backup bilge, btw) putting the plug into the sink - or a cork :-) - dramatically improves the suction.
Personally, I like a dry bilge - one of the advantages of an encapsulated keel. The only modification I have seen, which I liked (on Enchantress, last I heard just completed the crossing to Hawa'ii), was moving the grey water tank further forwards, opening up the entire space under the cabin sole hatch for a lot of extra storage.
You may want to look for the discharge thru-hull for the grey water tank - perhaps you never had one. For me, the discharge is above the waterline, behind the sink. If you never had one - well, I don't like putting more holes in the hull!
good luck