When a kettle of water is boiling, do native speakers say?
The kettle is boiling.
The water in the kettle is boiling. OR
The water is boiling.
Even without specifying a kettle of water #1 is the winner by a country mile.
We only say "The water's boiling", when the water is in a pan etc. Provided the water is in a kettle, we say "The kettle is boiling". I should add that, of course, it is not actually the kettle which is boiling, which means the statement is technically incorrect.