Friends of mine have installed a heating system consisting of fine black tubing that's carefully layed to cover several square meters of their roof. The sun heats the black tubing as the swimming pool pump forces water through the thin, warm, pipes. I believe that it's able to heat the pool by up to 10°C - which makes their winter-water about the same temperature as my pool's summer water!
While I know that sharks travel all waters, it's fairly common knowledge that they are more likely to frequent the waters of False Bay, visiting places like Muizenberg and Fish Hoek. Swimming on "the safe side" (i.e. in Table Bay) has its downside though, in that the water is a fair bit colder than the waters of False Bay.
If you do ever swim at False Bay's popular Fish Hoek beach, be sure to keep an eye out for the Shark Spotters' flags. The Spotters sit high up on the hills watching the ocean for the presence of sharks. They radio information to their colleagues on the beaches, who in turn raise one of four flags:
When a shark is spotted, a siren is simultaneously sounded, indicating that swimmers should stay out of the water. The only time you should really enter the water is when you see a green flag. If it's any other colour, it would be best to keep out of the water.
In closing, remember that if it's a beautifully warm day, and you notice that you're the only one swimming... there's probably a good reason. ;)