Water exists as a solid at or below 0 degree Celsius.
At 0 degree Celsius, water can go through a phase change, either freezing or melting, depending whether energy is given or taken away. If the temperature is below 0, water should exist as solid as the temperature is lower than its melting point. If the temperature is right at 0 degree, the water can partially be solid and liquid at the same time, during phase change.
Therefore, water exists as solid at or below 0 degree Celsius.