All those regular sharks arriving at once like The Cavalry to devour the bigger megalodon at the end seems like too-convenient timing. All of them appearing simultaneously, even though sharks generally aren't cooperative hunters? Sharks of several different species, to boot? But considering how slicing the meg's belly with the jutting metal from his sub's damaged fin was Jonas's opening move in their fight, that thing's been leaking blood by the barrel-full for some time. The other sharks had been gathering around it, attracted by the scent, all through the battle; they just hadn't been stupid enough to go near that gargantuan monster - one that'd surely gobble up regular sharks as eagerly as squid, whales, or Puny Humans - until Jonas destroyed its brain and the damn thing stopped moving. Soon as its nervous system shorted out and their electroreception said it was no longer dangerous: dinnertime.
Speaking of the smaller shark cavalry, you could argue that it's also a cheeky reference to the Megaladon's real world evolutionary history. When the Ice Age hit, Megalodon's primary food source, whales, were able to thrive thanks to their insulating blubber. But without blubber of its own, Megalodon was restricted to the equator, unable to follow its prey into the colder waters. Whales, naturally, stayed far away from the tropics in order to avoid their natural predator, and since nothing else was big enough to satisfy the giant shark's caloric requirements, Megalodon was starved into extinction. Meanwhile, smaller species of shark did just fine, feeding on small fish and the occasional seal, ultimately surviving the Ice Age. So who better to deal the final blow to the Meg than those who originally outcompeted it?
Why did the shark go after all the people on the beach, when they were too small to really be more than a snack? Well, what does the meg eat under the thermocline? Giant squid - animals with lots of individual moving parts/tentacles. It's possible that the meg 'saw' the movements of all the individual people and mistook it for something tentacled/squid-like. The whale song managed to lure it away because the meg wasn't getting enough bang for it's bite and/or it had decided it really liked the taste of whale flesh more than squid.
Note that when it does attack the crowd of swimmers, it's not going for individual humans at all: it's striking the party rafts (and that one idiot in the giant plastic bubble), because they're large enough to seem more like a suitable food source for a predator its size. It only starts zeroing in on the people when the rafts themselves prove to be inedible.
