Why is Saturn so much less dense than water?

Saturn’s low density comes from its composition and structure. Like Jupiter, it is a gas giant made primarily of hydrogen and helium, which are the lightest abundant elements in the Universe. Saturn’s large size does not make it dense because its mass is spread through a huge volume, much of it compressed gas and liquid rather than rock or metal. In principle, if there were a bathtub big enough, Saturn would float in water, though of course it would break up and be destroyed long before that could happen. Its low density is one of the most famous examples of how a giant planet can be enormous without being heavy in the way rocky planets are.