Would the sharks come after him that fast?
Yes, if he was flailing around and splashing the water like an injured fish................
Indeed, "shark-infested" is journalistic cocaine used to excite readers.
Unless one jumps into the middle of a shark-frenzy with an already bloody carcas in the middle, sharks are cautious creatures that will first circle prey, take some time to figure out what it is, confirm if its edible, make an exploratory bump/bite (which nonetheless could be fatal to humans) etc...
If the sharks associate a splash with food and there are lots of them there, I think the sharks would react very quickly because they want to beat the other sharks to the food.
Sharks mistook him for chum.
He does not look like the color that the sharks have been imprinted on.
Maybe they have been starving and had to eat 2nd choice.
Bull sharks. Nasty fellows. ...and stimulus/response to feed off the boats. Kid didn’t stand a chance. Kid who dared him got a heavy load on his shoulders even if final decision was his.