It's part of the Hungering deep peeps. The sharks didn't start appearing in the numbers they are now until right around the teaser.
But besides that, if this is a matter that stays, im perfectly fine with it and here is why..
ONE: Nature
Though admittedly great whites don't swarm in the same manner as some shark species, it's not unheard of. And considering the sharks of the Sea of Thieves to be special case, possibly influenced by the cursed nature of the sea, it's shouldn't be dismissed that this subset of great whites exclusive to the sea of thieves would adopted swarming behavior, either due to competition over a clear lack of species hearty enough to sate a GWs appetite, or even perhaps as "protection" against the "greater" beast of the sea.
TWO: Danger
Sharks should be feared everytime you dive into the water. It should be a risk. Before you didn't fear sharks too much because as long as you had one guy sniping up on deck, sharks offered NO challenge. Now for the first time there are more sharks than there are bullets in any one firing round, and therefore can't be dispensed as easily. This actually makes those shipwrecks a challenge now, forcing a greater number of your crew to take up shark duty while someone gets all the treasure. Those wrecks can sometimes come up with better treasure than even high rank voyages, so they did need more risk-reward anyway. Finally, there is also the matter of ship battles that essentially devolve into "underwater skirmishes" where both crews are diving into the water in the attempt to get an easy boarding onto the enemy crew and those awful underwater skirmishes that happen as a result. The fights should stay on the boats, like it should be. And swarming sharks is a fantastic "soft" limitation on reducing these unnatural tactics while making successful boardings to be reliant on either luck, skill, or ramming, like they were actually supposed to be.
Also fun fact.. ringing the belly wildy for a second sometimes instantly summons a shark swarm even without a player falling into the water. If you have a crew whos abusing the ninja pirate approach to sabotaging your ship this is a good way to essentially generate an area of sharks in the area the bells were rung.