Why is Saturn less dense than water?

Saturn’s average density is lower than water because it is mostly composed of hydrogen and helium, the lightest common elements in the universe. Although Saturn is massive, those gases make up a very large volume with relatively little mass per unit volume. The planet is also compressed by gravity, but not enough to raise its average density above water. In theory, if you had a giant enough bathtub, Saturn would float, though it would be torn apart by tides long before that happened.