Answer
By its temperature and pressure.
As an isobaric process where the temperature is reduced.
Work Step by Step
The state of the water can be described normally by its pressure and temperature. In other cases may be necessary to describe other properties as chemical composition, but in this case, the water in a bathtub can be described by its pressure and temperature.
The process that this water experiences as it cools is made at constant pressure, isobaric, and the temperature comes down.
