A single word for boiling water would be like the single word "slush" we have for ice in water.

It likely could apply to other liquids in the same mixed state, but would be assumed to refer to water (or solutions or colloidal mixtures primarily consisting of water) in common speech.

Water is extremely common, and has anonymously high heats of crystalization and vaporization, so it is the most common example of a mixed phase system and the only one most people encounter in everyday life.