The concept of a planet made entirely of matter in the liquid state has been considered, i.e., a liquid planet analogous to a gas planet made entirely of gas. Evaporation and freezing present challenges to the notion. The planet may need an atmosphere to "seal" it, and if the atmosphere is thick (e.g., half the diameter as viewed externally), then it is a matter of semantics whether it is a gas planet or a liquid planet.
Jupiter has been described as a liquid planet. Some current theories consider it to be liquid, possibly from the center, up to the surface we view.