When I visited Cape Town, looking for Halal Muslim food was very easy. This is due to the fact that about 10% of the population are made of Muslims. Other than that, there are some restaurants that serve only seafood.
If you are lazy to go find Halal Muslim food around the town, you can always go to Victoria Wharf’s Food Court at Victoria & Alfred Waterfront. There are quite a selection of Halal food there. There’s Nur, Anat, Subway and many more.
If you are not into Arabic/Indian/Middle East food, you can always opt for Texie’s. They serve Snoek and Chips, Seafood Platter, Calamari Gatsby, etc. Texie’s can be found all over Cape Town; there’s one on Adderley Street (just opposite Fountains Hotel where I stayed), there’s one at Sea Point, one at Grand Parade (near Cape Town City Hall). You can view the directory here.
And of course there’s huge selection of seafood restaurants around Victoria & Alfred Waterfront.