These cheap containers are invaluable if you have a vehicle, and are not taking a large cool box with you. They cost about N$50 and are available from some big hardware or camping stores. If you are on a self-drive trip, these are an essential buy in your first few days.
The water in Namibia's main towns is generally safe to drink, though it may taste a little metallic if it has been piped for miles. Natural sources should usually be purified, though water from underground springs and dry riverbeds seldom causes any problems.