Why didn't the mother jump in? Why didn't the boyfriend jump in? I'm not a hero but if my child--even my dog--fell in the water I'd jump in to rescue him. And have done so (for the idiot dog).
For all the years I homeschooled the kids and lived with them 24/7 (at that age) I never would have left a child unattended by the water like that. It was totally irresponsible.
Simple: You are a healthy, sane human.
Those parents are inferior animals.