Dont worry about the cold water in Cape Town.
You spend most of the time onto or in the air.
Besides not all of Cape Towns water is cold.
Kite surfers from all over the world come over for the season. Besides.. if the worlds best kite surfer Aaron H from the UK chooses to train in CT over summer, it should be a good spot.
If you find the water cold then head up the coast. There are plenty of great spots all the way up IE Durban.
Great wave ridding all the way !!
Cape Town has a very good flat water spot called Langebaan. The water is warm and conditions are AWESOME.
Attached are some of Cape Towns good kite surf spots.
There are plenty more.. just explore.