I don't usually accuse musicians of faking their work, but this seems fishy. The way the guitar responds to movement doesn't scream "filled with water", the guitar balances much too well to have any decent amount of water in it, nothing about the guitar looks wet, the top sounds like it's resonating fully, the swishing doesn't sound right (sounds more like something being dragged in water rather than sloshing), and the fact that he said it destroyed the guitar make it seem a little fishy. If there's water in it, it's not nearly as much as the opening clip would make you think.
I'm gonna fill up an old epi acoustic I got for cheap when I get home to see if I'm just being a hater or not.