If your vehicle is set up for partial submersion (snorkel, and breather tubes for engine breather, diffs, etc), then you might be able to create some inflatable pontoons that attach to the sides of the vehicle and using them as basically floaties for your car. Car would obviously need to be light enough that the displacement of the floatation devices exceeds the vehicle weight. Such a device could be inflated with an on-board air compressor and could potentially scale up or down (large rigid truck or ATV). Propulsion through the water could be by whatever means you have, anything from a paddle to a pto driven propeller with steering, or even pulling yourself across the river with a cable (would require someone on the other side, or a swim). I imagine this has already been invented, or is totally infeasible, though.