Saturn is mostly gas. Would it float in a big enough bathtub?

Answer

Saturn's average density is about 0.69 g/cm3, less than water's 1.0, though no tub could hold it.

Context

Saturn's average density is about 0.69 grams per cubic centimeter, against 1.0 for liquid water. It is the only planet in the solar system with a mean density below that of water; Jupiter, Uranus and Neptune are all denser than water, and the rocky planets are several times denser. The low value reflects composition: Saturn is made mostly of hydrogen and helium, the two lightest elements.

The comparison is a thought experiment, not a realistic scenario. Saturn is about 95 times as massive as Earth and roughly 116,000 km across at the equator, so no bathtub, or ocean, could hold it, and its own gravity would pull a water tank apart. Average density also hides structure. Deep inside, the pressure compresses hydrogen into a dense fluid, and a core of heavier material, whose mass is uncertain, sits at the center. Only the outer layers are thin and light.

Saturn is also visibly flattened. It spins once in roughly 10.7 hours, and because it is a fluid body, the equator bulges so that the equatorial diameter exceeds the polar diameter by about 10%. Gravity measurements made by Cassini during its final orbits in 2017 helped constrain the interior and suggested winds that reach deep below the cloud tops. None of this changes the basic point that a planet of this size can be less dense than water.