Saturn
A planet's weight does not determine whether it can float in water. It's density is what determines that. Your question should be "Which planet's density is so small that it could float in water?"
Saturn has a very low density and it would be able to float on water. The density of Saturn is: 0.687 grams per cubic centimeter.
Saturn would float in water if you could find a big enough tub and the water to fill it.
The planet Saturn could theoretically float on giant planet like Jupiter because Saturn has a lower density than Jupiter due to its composition being mostly gas and liquid. This would mean that Saturn would have the ability to "float" in the atmosphere of Jupiter, similar to how a boat floats on water.
Saturn is the planet that is so light that it could float on water. Its average density is less than water, which gives it the unique characteristic of being able to float in a large enough body of water.
No, the density of Uranus is 1.27 g/cm3 (the density of water is 1.00 g/cm3), therefore Uranus would sink. The planet that would float in water is the planet Saturn which has a density of 0.687 g/cm3.
Saturn.
Saturn is the least dense planet, only 0.7 g/cc on average (less than water's density). If you could put Saturn in bathwater, it would float.
Saturn is the planet with a lower density than water, causing it to float if there were a large enough body of water for it to be placed in. Its average density is less than that of water due to its composition of mostly hydrogen and helium gases.
The planet Saturn
Saturn could float in water, because it has a density of 0.7 that of water. That is, however, a theoretical construct, because you would need an enormous pool of water, with a gravitational field much larger than anything we have, in order to achieve that state.
Saturn is less dense than water. So the Answer is Saturn.