Well yes, but there is no way it could use only gravity, there is some pumping going on somewhere, somehow otherwise it could do the job. Certainly, without much production anyway. For gravity to do the job the water/oil to be separated has to be at some elevation above sea-level, how does the water get there?
Where do I say the ships don’t use pumps? They obviously both use pumps, but for the separation the ships use gravity, and that uses less energy that centrifuges.