You'll most probably try a few different restaurants during your trip. Local restaurants are generally inspired by cuisine foreign to Southern Africa including Portuguese, Italian, Greek or Japanese food. Good quality and reasonably priced pizzas, pastas, fish & chips and sushi are often widely available. Most restaurant menus include a good variety of food options.
Vegetarian meals, although somewhat limited, are always available.
In Namibia, local restaurants are often inspired by German cuisine. Seafood, game meat and chicken are also very popular there.
In South Africa, typical local meals (bobotie, vetkoek, bredie, walkie-talkies...) are rarely featured on menus. On the other hand, braais (barbecue) are extremely popular and, if you have booked a guide with your trip, you will no doubt experience this several times!
Restaurants in Southern Africa, have to abide by strict hygiene rules so food poisoning is fortunately very rare and not something you should worry about.
Tap water can be of good quality in certain areas. Whether you can or can't drink it will generally be mentioned. In doubt, ask reception staff at your accommodation or simply drink bottled water. Ice cubes are usually made of mineral, prepared or filtered water so no need to fear anything.