More to the point, even if the vehicle can seal completely and keep the water out, very few bodies of water that deep would be any safer to traverse in a car for other reasons. Most significant of these I think is the force of water pushing on the vehicle laterally. Claiming that a consumer vehicle can ford rivers or creeks up to 31 inches deep WILL get people killed regardless of how well the designed the vehicle. Don't drive through flowing water or even still water through which you cannot clearly see the bottom unless you're prepared for things to go very badly very fast.