Final answer:
Saturn is the only planet in the solar system with a density less than that of water, making it less dense than Jupiter and capable of floating in a hypothetical water bath.
Step-by-step explanation:
The planet similar to Jupiter in size but with a density less than that of water is Saturn. Saturn is about 25% as massive as Jupiter, but when you calculate the density of Saturn, you find that it is less than the density of water. While Jupiter has an average density of 1.3 g/cm³, Saturn's density is even lower, making it the only planet in our solar system that would float if you could find a bath big enough to hold it!