The quality of tap water in Namibia is quite good and perfectly safe to drink. Many of the lodges in the more remote areas will make use of water from a borehole which is also safe and healthy to drink.
If you are on a self-drive safari in Namibia and traveling in a hired car and plan to camp then it is recommended that you buy bottled water which is widely available at shops and fuel stations. Buy several 2 litre plastic bottles and use this as your drinking water if you prefer. This water can also be used to boil your water for coffee and wash your dishes. Always ensure that you have more than enough water when traveling in Namibia especially in the more remote areas.
Is it necessary to bring water purification tablets? The answer is no! However there is no harm in bringing them with you. Remember though that even the mildest of the local microbes may cause slight upset stomachs for an overseas visitor.