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Things float in water if they are less dense than water. You calculate density by measuring the mass of something and dividing by the volume. Water's density is 1 gram per cubic centimeter, so if a planet's density is less than 1 g/cm3, it would theoretically float.
The only planet in our solar system that is less dense than water is Saturn. If you're limited to the planets in our solar system, this is the most common answer.
However, some people will get technical on you, and calculate how much water it would take to float something that big. There's a fun article in Wired that shows how hard it would be to actually float Saturn. (See the link below.)
If you can consider planets outside our solar system, consider HAT-P-1, which is an exoplanet about as dense as cork. It would float more easily.
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