A while ago The "History Channel" had a show about early 4wds. In one scene they showed a bunch of soldiers unfolding a large tarp in the water. One soldier then drove a Jeep into the center of the tarp and the rest of them tightly folded the tarp over the top of the Jeep. 4 or 5 soldiers then got on top of this package and paddled the now floating vehicle across the water.

As they say, necessity is the mother of invention.