Saturn is less dense than water, although no ocean could ever hold a planet.

The longer version

Density compares mass to volume, and Saturn is composed largely of hydrogen and helium with no large rocky fraction. Spread across its enormous volume, that material gives an average density lower than water, even though the planet is immensely heavy in absolute terms. Gas giants in general are far less dense than rocky worlds.

The comparison is a thought experiment rather than a possibility. Any body of water large enough would collapse under its own gravity and the planet's deep interior would deform and mix with it. Inside, pressure rises so much that hydrogen gradually behaves like a liquid metal.

Where this entry sits

The night sky looks calm, yet almost everything up there is moving, burning, or slowly changing. These facts cover planets, stars, the Moon, gravity, black holes, comets, and the story of how people began exploring beyond Earth. The full list sits on the Space and Sky page, where every entry is listed in order.