There are three options I know of:
The shipwreck Tour. It starts at Luderitz through Diamond Area 1 and follows the coast, focusing on the shipwrecks. This is my first choice, but I have received no response to phone calls or e-mails.

The Faces of the Namib. This is a shorter tour that starts in Soussus Vlei and goes to Meob Bay and Walvis. It is more of a play trip, with camping in one spot for more than one day and doing circle day trips, IRC.
The Luderitz to Walvis Baai Live the Journey. What we did. 600-650km. 6 days, five nights. It starts inland from Diamond Area 1 and goes in and out from the coast, visiting abandoned vehicles and towns. You sleep two nights near or at the coast.
I got close to 2 km/l for the trip... I took 295l and ended in Walvis as the light came on.