It is objectively a phrase, and not a word, because you can substitute the water for literally any other liquid and form a perfectly coherent phrase. boiling oil, boiling syrup, boiling coca cola. "Boiling" in this context is just a participial adjective, modifying the noun "water". If "boiling water" is a word, so are "six men", "good idea", "large rock", "7 year old boy", "Californian trees", "metallic flooring", etc.

Better yet, you can take advantage of English's adjective ordering to demonstrate this point. Would I describe the water I'm currently boiling for the purpose of cooking "cooking boiling water", or "boiling cooking water". Since purpose tends to be the last adjective we use, any native speaker would choose the later.