Yep ok then.
When I saw it done they put a toughened glass platform in the pool from one side to the other with glass supports underneath. The top platform is about 2 inches under the surface of the pool. Toughened glass will support loads of weight, you could have had two or three people on the platform.
There are gaps under the platform so people can swim underneath. If you look closely you will see that the surface of the water is constantly moving, this is to stop you seeing the glass platform edges.
As with most things like this it is an illusion and all the people in the pool are in on it.
Don't it look good tho.........
They use this method in several music videos I've seen in the past, can't think which ones they were now.........