The correct answer is - the air.
The Namib Desert is one of the driest places on the planet and that is due to the cold and dry ocean currents alongside the southwestern part of Africa which are not providing the right conditions for precipitation to occur.
Even though there's no rain, because the Namib Desert is very hot, and the ocean currents very cold, the end result is mist that appears every morning, as well as dew.
The plants living in this desert have evolved in accordance to the conditions, so instead of being dependent on their roots for getting water, they get the majority of the water through their leafs and skin, directly from the air.