Why it is necessary for instructors to make such a song and dance about a simple, perhaps indiscreet, piece of fun flying, carried out by a pilot who appears to have flown the manoeuvre successfully, really baffles me.
This instructor has 2 dead friends, one who I taught to fly, who gave into to the temptation for a "piece of fun flying"; and paid the ultimate price......
With respect to watersking I have, of course never done any, but I have owned 2 float planes and taught pilots for the float rating. Working off the water is never a sure thing and most of the float course is about learning to evaluating the water conditions and working to recognize and minimize the inherent risk.
As for the video, the guy taking the video is standing next to a runway, and it sure looks like the watersking was planned to maximize the "look at me I am such a Sky God" factor.....