The highest possible temperature that can be reached by water, if all its molecules were to turn into steam and escape from the container holding it, would be the critical point temperature of water.
The critical point of a substance is the temperature and pressure above which distinct liquid and gas phases no longer exist, and the substance becomes a supercritical fluid. For water, the critical point occurs at approximately 374 degrees Celsius (647 degrees Fahrenheit) and 22.1 megapascals (MPa) of pressure.
At temperatures above the critical point, water exists as a supercritical fluid, which has properties of both a liquid and a gas. It doesn't have a distinct boiling point or condensation point like a typical liquid-gas system. Instead, it transitions gradually from a liquid-like phase to a gas-like phase as temperature and pressure increase.
Therefore, if all the molecules of water were to escape from the container, they would exist as a supercritical fluid, and the concept of a specific temperature for the system would no longer apply in the same way as it does for the liquid or gas phases.