Namibia is a desert county containing not one but two desert, the Kalahari and Namib Desert. Reading between the lines even our more dim witted readers can therefore probably work out that water is in short supply in Namibia.
Although tap water is pretty much always drinkable in larger town and cities such as Windhoek it is often recycled and so can taste quite chlorinated. Most lodges and guest farms in rural areas use bore hole water which is clean and fine to drink. That said many people choose to drink bottled water which is easily obtainable at supermarkets around the country.
Of course the act of drinking water is completely superfluous as Namibia has awesome beer and access to fine wine & gin - so really your major concern should be using water for the purpose of washing. If you do feel the need to wash regularly please remember that water is scarce so take a short shower and save the planet!
Water, taken in moderation, cannot hurt anybody. Mark Twain