The planet 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?"
So light it would actually float on water.
Saturn has denstiy so low that it could float on water.
The low density of Saturn would allow it to float in liquid water.
Saturn would float. It is the least dense planet, with a density that is lower than the density of water.
It is either Saturn or Neptune... though you may want to do more research on the subject ;)