There gotta be some truth to this. Most vehicles will actually float, just depends on how long. FWD cars usually sink the fastest, with all the weight in the front. A well sealed truck could stay a float. About a year ago, Oklahoma had severe rains and floods. One couple drove their big black Dodge truck on flooded roads. Amazining, the truck stayed afloat until they bailed out of the windows and then it sank quikly.
Its all a matter of displacement and being well sealed. To help the front end stay floating better, you could easily place a empty sealed container along the inside of the front bumper, and weather strip the tailgate.