It looks like it happened at the famous speed sailing spot on the island just North of Pozo, called Arinaga.
This is Robby Naish breaking the World Kitesurf Speed Record there in 2003.
www.dailymotion.com/video/xgzs94I have read that diving is prohibited there and that may be the case because it has been used a lot over many years for speed sailing.
There are rare places that have natural conditions that suit certain sports/activities and maybe many, many spots that suit other activities.
While swimmers are not a risk to others, there are many cases where areas are designated for certain activities and not others. So although on safety grounds there is the argument that swimmers should be allowed to swim anywhere and others have to then leave the place, designated areas are a better way of dealing with some situations.