Watch it again - the Magic's Greatest Secrets guy showed how to do this with gaps between seperate perspex tables (this was in a swimming pool). Check the polis, but. They peer over the edge as they're in front of him (para they'd hit it). It's a collection of perspex "tables" - specifically made for the Thames with its depth and current and the likes but it's perspex.
No such thing as magic, but I heartily believe in illusion. Nearly had a stand up square go with my mate's family who believed David Copperfield made the Statue of Liberty vanish. They actually thought it had disappeared and then re-appeared when the trick was finished. Would not even entertain that there was any camera work/mirrors etc. That Breaking The Magician's Code showed how it was done with mirrors to make an elephant disappear. Case closed. Not a fan of walking on water (atheist) or walking through the Great Wall of China but love the card tricks, coin tricks and close-up stuff David Blaine does. Even that twisted midget Paul Daniels was excellent at the sleight of hand stuff (Bunko booth? Don't go there!) I've never seen better than David Blaine with his street magician stuff, he is an expert, but why does he think standing on a pole or sitting in a plastic box is anywhere near entertaining? The most entertaining thing about his perspex box over the Thames was when someone started hailing golf balls at him!