Some people said that there's something in the water, because they can stand up, the depth of the water was shallow
Unfortunately, the answer could be a lot simpler than unseen predators or inability to swim in shallow waters. It looks like one of them, possibly the girl, might've stepped into an underwater drop-off and instinctively went into panic mode. Being pulled under by a downing person is common, since they're little more than a flailing animal at that point, grabbing anything they can. It's also harder to escape than it looks, many rescuers have died that way
When I was younger I almost drowned in a pond that was a lot smaller than this, the water was up to my waist. One second everything is fine, and then the next its like a bottomless pit pulling me down. I couldnt keep my head above water, infact there was maybe an inch of water between me and the surface, so I could almost tast freedom. That was the worst part, seeing a way to live just out of your grasp, the sheer amount of terror and panic is something I have never felt since. Hell, Im certain that something had my leg and was dragging me down but that doesnt even begin to compare. Eventually I was in there for so long that I remember sinking, watching the light fade and the world turn black right before being dragged out by a family friend. The about drowning is that its painful, your lungs fill with water and all that. But once you get tired, and suddenly not enough oxygen is getting to your brain the pain goes away completely. You feel sleepy, and safe, you just accept that your gonna die without any resistance. After that I learned how to swim and now I can tread water for almost an hour without using my arms. I still have nightmares qbout that day fairly often, and the whole thing plays through my head almost everyday forcing me to relive it.