Isn't it possible Kiely made her way to the beach intentionally? Perhaps while there was still moonlight enough?
Perhaps parking was haphazard and it made sense to her to ho that way -- if only to turn around. Perhaps she was trying to avoid a loggerjam of vehicles and had a different route mapped in her head. Perhaps what she did not anticipate was the utter blackout of moonset, around 12:15-12:20, as I recall. Nor the high water level/short beach. If she found herself suddenly in even just 2 feet of water, she was already in trouble.
If, once in the water, she had to take a long moment to search in the glovebox for an implement to break a window, it was already too late. (The guests who were in the car early, while recharging batteries may have left the Truly can, may have requested the window half-open -- not actively on her mind when she's in the reservoir, sinking and thinking fast.)
I think this is just the saddest of sad accidents, sadder still that Kiely had the presence of mind to try to save herself but couldn't
She just ran out of time.
JMO