Something about this doesn't add up for me. I understand how an accidental drowning can occur anywhere, we live near a deceptively difficult river and there are kayaking and swimming accidents all the time. But would you take a 4 year old out alone on a motor boat into a 130 ft deep lake? With a group or even a friend, sure, but by yourself? How was he going to the bathroom? Even with a life jacket on I don't know I'd go swimming in the middle of a deep and difficult to see lake with a toddler.
The article posted above said her purse was found in her car. Was her phone in the car or on the boat? Did she have a day bag on the boat with her son's various things he'd need for the day? I think that will say a lot. I hope she was ok, and I'm relieved her son was found unharmed.