Saturn is the second largest planet in the solar system. However, it is composed primarily of hydrogen and helium and lacks a solid surface. The density of Saturn is 0.687 grams per cubic centimeter. This density is less than that of water. Therefore, it is the only planet in the solar system that is less dense than water, which has a density in the range of 1.0gm/cc if it is 39.2 degrees F or as little as 0.95865 gm/cc if it is 212 degrees F or boiling. Therefore, due to its low density, theoretically, Saturn could float in a huge amount of water similar to the way ice floats in a glass of liquid water. An interesting fact--Saturn is the least dense of all the planets in the Solar System; however, its mass is greater than all the planets except for Jupiter.
Even though Saturn is much more massive than Earth, this mass is spread throughout a much larger volume than Earth and so Saturn is less dense than Earth. Saturn is the least dense of all the planets in the Solar System! Saturn’s average density is only 0.69 g/cm 3 , which is less than that of water (1.0 g/cm 3 ). This means that a volume of water equal to the volume of Saturn would weigh more than Sat-urn does. So if we imagine a titanic tub of water big enough to hold Saturn, Saturn’s weight would be supported by the water and Saturn would float! Of course, when the plug was pulled and the water drained away, Saturn might leave... a RING!
It's just like how a boat can float and so can people but penny's cant.