Cars float well enough to lose traction. I've made water crossings in streams, but that is far different than crossing a flooded roadway. Road surfaces become slick, and flood waters can move extremely quickly and have a lot of power. If you've ever hike across a thigh deep running stream you will be shocked at how powerful the mass of water is. Never cross a flooded road with running waters unless you want to make the news posthumously. A deep puddle (non-moving water) in the road is a different story, but I would still not cross if the underbody will be in the water.