Yes, there is a limit to how hot water can get before it undergoes a phase change. The maximum temperature water can reach in the liquid state is the critical point of water, which is approximately 374 degrees Celsius (647 degrees Fahrenheit) at a pressure of about 218 atmospheres.
At temperatures beyond the critical point, water transitions into a supercritical fluid state, where it exhibits properties of both a liquid and a gas. In the supercritical fluid state, the distinction between liquid and gas phases becomes less relevant as the density and other properties change continuously.
However, even in the supercritical state, water does not reach temperatures as high as 1 million degrees Celsius. Such extreme temperatures are far beyond the capabilities of ordinary matter and are closer to the conditions found in astrophysical phenomena like stars.
When considering temperatures of 1 million degrees Celsius, the behavior of water would be drastically different. At such high temperatures, the water molecules would dissociate into highly ionized plasma. The electrons would separate from the atomic nuclei, resulting in a gas-like plasma consisting of charged particles.
This scenario is not applicable to water in its ordinary state. The conditions required to reach temperatures of 1 million degrees Celsius are extreme and involve environments that are far beyond our current technological capabilities.