Back in Harry's day they used glass walkways under the water. Since the glass, disappears in it same refractive index yada,yada,yada
Blaine and others now a days use plastic sheet, they prefer slightly darker water so to hid the frame work from any flash of sight. It is the same deal the refractive index is the same. The kayaks can slide over the plastic sheeting with a very little water over it it is probably deeper in those areas by a little bit.
For the shop window the key is in two things, look at the header above the window that large area above the window. 1 Holds the glass as it is slide up enough, so he can slide snake style under neath it. 2. The Crowd of people on the outside of the shop window are hired shills (actors)
to show the proper reaction and not give away the gaff. Biggest deal is to find a store/shop to let you rig the window. The store can even be fake just rent a shop fill it with rental props, and crowd boom the magic of TV. Since even today most people still think like the internet if it's on the tube its real!
That way they can say that "There are no camera or editing tricks used in filming this" They never say that every other thing you see is controlled. Blaine was famous for having everybody with in camera shot on the street, in his employ along with the lighting and camera angles.
Here's a free bee Blaine's famous street levitation, is accomplished by the use of a harness and a crane parked on the next street, with the boom over the building. And camera angles along with it is always a 1 or 2 story building.
As somebody once said "It's all done with smoke and mirrors" The three biggest expenses in doing large scale magic is the costs of the Engineers, mechanics, machinists and building the prototypes till it works.
Now if you ask about smaller scale tricks "I know nothingggggggg" as Shultz use to say
