I have a preference for no seatbelts as if things go pear shaped as I don't think there's enough time to get the harness belts off the kids. (think about getting your own belt off, partner doing the same, trying reaching around and release the kids clip or find your seat belt knife, cut the belt without injuring the kid, all while the car is sliding and water is rising)
My reasoning behind no seatbelts is;
* a water crossing will be done at low speed, enough to make a bow wave.
* if things went wrong and I wanted to bail it would probably be from a cross current about to sweep the car away.
* grab kid(s) by chest-clothing and take them with you through the front
Windows should be down, don't need to be all the way.
The biggest part would be, if the risk is high and there's no real need, I'd give it a miss anyway.
Awesome topic though