Okay, so imagine you had a really big bathtub. No, bigger than that. We need to fit an entire planet in there, and not one of the small ones either - Saturn! If you can picture such a thing and fill it with water - and if you can calculate how much water you'd need you're more gifted than us - you may be surprised to hear that Saturn would float if you placed it gently into the tub.
Saturn is the least dense of all of our neighboring planets; it's entirely made up of hydrogen and helium. Because of this, it has less density than water on Earth, meaning we can be certain of its floating properties. A beautiful world to look at, famous for its elegant rings, we believe the core of the planet is made up of metallic hydrogen but can't be sure, because the phenomenally high temperatures and pressures needed to create it can't be replicated on Earth in order to study.