Kite surfing.
You can see the surfer that is holding the kite at some shots.

Personaly I don't think it is real though
would the person filming this not try to follow the side it went? Okay I must say I rewind it to see wich direction it gone. It was fast.
But would there not be noises of the person filming it? like in 'wow' or 'what the' sounds of him? Awfully quiet, except you can hear the ocean clearly. The cape is usually full of people on such days. So other people should have seen it and this would not have been kept quiet.

As Ian think it is kite surfing, it may be the other things flying around or it could be birds, but that thing that standed still (the ufo) was definately not a kite nor bird and it disappeared a bit way fast for something normal. This is my anylizes on it

It is great to see my country (south africa) comes on UM too! whoohoo!!