Current water temperature is about the same as an average UK indoor swimming pool, so I wouldn't have thought that cold water shock would be likely.
Not suicide either, surely she wouldn't do it in that way and leave her DS alone in a boat?
I suppose they'll have specially trained officers/social workers questioning her DS to see if he can tell them anything, such as whether he had actually been in the water that time himself, or if she could have fallen in. But then you'd think she'd be able to get herself out, or at least summon help from someone else on the lake if it was a popular leisure spot.
However, the wikipedia page lists several drownings, many of them in summer. Safety information about open water swimming always mentions cold water, risk of entanglement and underwater currents as risk factors.