If your tank was well set up correctly with ample reeds & hiding places your turtles would stay in the water overnight, by far, the majority do, which I presume you have heated. The fact if there is a basking light on or off should only have minimal impact on the water temp, as this should be maintained seperately. The hatchling turtles will then spend their nights in the water, not allowing it them dry out.. They do this in the wild, for safety, an instinctive self defence reaction.
Your little ones on the dock overnight may have been dehydrated, so not too far from death at all.
Do you realise that the change to the law in regards to legal turtle size was 1/10/09, seeing that it is now 20/10/09, I really think you are streaching it a bit Dabool with your comment "they have had a 75% mortality rate with hatchling turtles since the laws changed" Hatchlings always have a high mortality rate unless they are raised to a suitable size & strength before they are sold.
You might want to ask Craig Latta yourself to verify if what I have told you is correct or not, you know, the guy who wrote the caresheet you like so much, the website where you got it, etc, etc.