Without knowing the circumstances (such as how long the flood water had been there, if any warnings had been given to drivers, what signals were showing etc etc), could it be that this was the first train to pass through the flood - hence the driver not knowing and not slowing down beforehand?
Sounds likely
DB» and welcome from me too (I have not been here long either and they are really very patient and helpful folk you will find, best of all, knowledgeable).
I wondered whether, as the tops of the rails were above water, whether there was the slightest risk of a derailment?