The truth about Cape Town water
The Atlantic Seaboard, Camps Bay, Clifton, Llandudno, is stunning and freezing year-round. The water is 12 to 16°C. You swim in it for the story, not the comfort. The False Bay side, Muizenberg, Fish Hoek, Strand, is warmer (Indian Ocean) and more swimmable. Pick your beach by which ocean you're willing to stand in.
The Atlantic Seaboard
Camps Bay is the postcard. Palm-lined, with tidal pools and beach clubs; busy and beautiful, cold water. Clifton is four coves, no wind, the wealthier beach; pack light as there's a stair descent. Llandudno is the prettiest, the least equipped, good for sunset. All cold; all worth it for the view alone.
False Bay
Muizenberg is the learn-to-surf beach, warm enough to swim, and the surf-culture heart. St James and its tidal pool are calmer, kid-friendly, and warmer. Fish Hoek is a long, family-friendly sweep with a catwalk and (in season) shark-spotter guards. Boulders is swimming with penguins, cold but magical.
The Southern Cape, warmer still
If you'll drive for warmth, the Whale Coast (Hermanus, Grotto Beach) and the Garden Route (Plettenberg Bay, Keurbooms) are where the water finally feels like swimming. Two to three hours from the city; worth a night or two.
The tidal pools
Cape Town's tidal pools are a whole category of their own, and one of the best free things in the city. St James, Dalebrook, Woolley's, Milton's. They're calm, they're warmer than the open sea, and they're the places locals actually swim when the ocean is too cold. Bring goggles. A tidal-pool tour is a better Cape Town morning than most paid attractions.
What to know
There are no lifeguards on most beaches in winter. Great whites are real on the False Bay and Whale Coast coasts. Swim at patrolled beaches, heed the flags, and don't swim at dusk. The cold is the bigger daily risk than the wildlife. A short swim and a long warm-up is the Cape way.