All the Slash fun requires is a set of aftermarket paddle tires intended for sand, Traxxas' optional high-speed gearing, and a Power Cell 3s LiPo battery. Then the Slash is fast enough and can move enough water to keep from becoming a submarine. As you see in the video, the Slash can even do jumps on water, land on a floating 'barge,' then keep on rolling over the surface. Traxxas did install some foam noodles under the body, however, because you can't totally rely on physics when you drive into a lake and go all X Games.
True, people have been driving all kinds of non-marine vehicles on water for years – and we don't mean engineered amphibious vehicles. This guy does wheelies on the water on his snowmobile, this guy took his tube-frame buggy on paddle tires across the pond, as did Top Gear's Richard Hammond, and Jamie on Mythbusters did it with a motorcycle on road tires, until he sunk. But with a Traxxas you can do it from a distance, stay a lot drier, spend a lot less, and do jumps.