This sort of behavior is baffling for several reasons. Firstly, striking/attempting to strike any living creature is obviously problematic, as jumping on top of a swimming shark - big surprise - can cause serious health problems for the shark. The shark could have experienced "tonic immobility," which is what happens when a shark is flipped over and enters a state of involuntary temporary paralysis. In tiger sharks, tonic immobility can be induced merely by placing hands on either side of the shark's eye area.
And, though the friends are presumably joking, calling a shark a "man-eater" and saying that it feeds on human flesh is perpetuating an unfair stereotype about sharks. In actuality, sharks rarely attack humans and do not find humans to be particularly appetizing. On their website, National Geographic News states that:
The United States averages just 16 shark attacks each year and slightly less than one shark-attack fatality every two years. Meanwhile, in the coastal U.S. states alone, lightning strikes and kills more than 41 people each year.
Tiger sharks are also listed as "near threatened" throughout their populated areas.
Though it is unclear whether these men will face any sort of repercussions for their actions, it's important to understand that it's really never okay to "prank" an animal in the wild, or attempt to jump on a wild creature. Not only can it be dangerous to your safety, but it can also be dangerous for the animal and cause them stress.
Just think of the young man who decided it would be hilarious to "surf" on a floating whale carcass, which was hosting several feeding sharks - an action which The Department of Fisheries deemed irresponsible and risky.
In short, the best lesson to be learned from all of this is that if you're ever on a boat out at sea and you notice some marine life in the water, please do animals everywhere a favor and keep your hands (and body) to yourself.