Would Saturn float in water?

Yes, in theory. Saturn's average density is lower than water's, the only planet in our solar system where that is true, because it is made mostly of hydrogen and helium. Given a big enough bathtub, it would float.

What's actually true

Saturn is less dense than water. Saturn is huge, but it is made mostly of hydrogen and helium, the two lightest elements. Its average density is lower than water's, the only planet in our solar system where that is true. In theory, if you could find a bathtub big enough, Saturn would float in it.

Takeaway: Big doesn't mean heavy for its size: Saturn is mostly light gas.

Why people believe it

Size misleads us: Saturn is huge, so it feels like it must be heavy, and in total it is very massive. Floating depends on density, though, which is mass divided by volume, and most of Saturn's enormous volume is light gas. The bathtub picture is a thought experiment, not something that could happen, but it is a memorable way to show that big and dense are not the same.

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