A 5 minute walk at most. There are plenty of other companies doing trips but Moby Dick Tours are really experienced and the cheapest. May I suggest if you are into forts/castles then take a look around the old fort, only costs a couple of euro and well worth it, it is only about 5 minutes on from the Moby Tour office along the promenade.

Moby dick tours right under the amphitheater in the port, not sure but believe this is where the cruise ship docks, making it about a 3 minute walk. Port Delgado not that big, walk from one end to the other in 10 minutes, check out Google maps, has Moby dick tours marked.