She hired the boat around 1:00 pm, for three hours. When she wasn't back by 4:30 the staff did go looking for them.I haven't read anything about the hire place keeping an eye on people who hire boats from them. I would have expected them to do so. I believe she had booked a 4 hour slot, and presumably the hire people do go out to call people back in who have passed the time slot?
She seemed to be fit and well, so would expect her to have been able to tread water for a decent amount of time, even 4 hours.
I also would be very surprised if she did make a mistake and not be able to get back onto the boat. Reports say she grew up with 20 miles of the lake and it was somewhere she liked to go. This surely wasn't the first time she swam.
I don't know if she didn't wear the lifebelt to swim, but I would think it would be harder to lift her son onto the boat whilst wearing one. Without the lifebelt, it would be easier, so she may have taken it off for that reason.
I don't know how experienced a swimmer she was, but reportedly most of the people who have drowned in that lake were not good swimmers.
If she was a good swimmer she may have gone into the water without the life jacket on.
Maybe she did not expect the boat to float away, but since it wasn't anchored the wind could have caused it to move quickly. She may have had to help her son get closer to the boat using one arm, which could have worn her out. I've done this many times with my daughter just to get out to the "float" on the lake and I can't imagine being able to tread water even for another 20 minutes.
Then, once she lifted him up onto the boat, the current could have carried the boat even further and she wasn't able to grasp onto the boat and went under.