Certainly nothing is going to float until the level of the outside water has reached a point where it has displaced enough air volume inside to balance the total weight of the vehicle. In any full chassis 4WD wagon I would guess that is above the base of the windscreen.
I know for sure that a 70 series trayback with a bit of gear in the back doesn't float at all, I don't know how much water came past the door seals before the water reached the roof outside but it just rolled down the dam bank and all we could see was bubbles and a few floaties to indicate where it was.
