Only one planet in our solar system is less dense than water: Saturn.
Saturn is a gas giant, primarily composed of hydrogen and helium. Its low density is due to its gaseous composition and its large size, which spreads its mass over a vast volume.
While the other gas giants, Jupiter, Uranus, and Neptune, are also less dense than water, they have a higher density than Saturn.
The terrestrial planets, Mercury, Venus, Earth, and Mars, are all denser than water and would sink if placed in a giant ocean.
It is important to note that placing a planet in water is a hypothetical scenario, as it would be impossible in reality.
