Evolution. Hell yeah :D
The cool thing about Evolution is that as long as there is a single way of doing it, there is a path for Evolution to develop it naturally in an Organism.
So let's get this bread.
I think you could take a similar approach to what many people propose for real Dragons with fire. That is that they mix two Chemicals outside of there body that catch fire. Or in other words a Exotherimc reaction. In the case for Fire Dragons, those Chemicals most of the time cannot just ignite in the Body because of the lack of Oxygen. But for us, that dosnt matter to much. We dont want fire. Just heat.
This problem has two sides. The water and how the Water gets hot.
I guess the easy problem is where the Water is. In the body. Idk be creative.
How do we get the Water hot ? Well, that is a problem. Hot Water tends to get less hot very fast. So my guess is that the Water would get heated inside the Body and released via a Valve.
Why? Well, you know how the Boiling point of water sinks if the Pressure is lowered? Yeah, my guess is that this Animal would have two Chambers. One in which the Hot Water + Vapor can rest. This Chamber would be really well, insulated with a minimal amount of blood since that would carry away heat.
The Other chamber would be at a lower Pressure. An Organ Similar to a Lung could push the stuff from Chamber A to B.
Now with the smaller Pressure in the one Chamber, we can get away with a less crazy reaction. My guess would be some sort of strong Acid. The Water could easily be saturated with the Acid and Acids have this nice property where they are really shitty at destroying certain Materials. These Materials also tend to be on the stronger side which would help lower the Pressure even more.
Nice. The current Animal has a two Chamber system which uses Acid to heat the Water and stores the stuff in a seperate Chamber.
In theory, this already works. Although the Hot Water would also be very saturated with Strong Acid. Which i guess is not a problem considering when this would be used. And again, Acid is really bad if you are not shielded from it but look at our Body. The Acid in our Bodies is not some weak stuff. Yet we are here just chilling.
Since my English tends to be a bit confusing, here is everything in a TLDR:
The Acid Chamber is deep inside the Body and has a lot or Small Openings around it. These on the one side suck Gases out but can also push Water and Acid in. The Acid Chamber is at a very low Pressure so less Acid has to be used.
A Lung like Organ (Maybe with a Tree like System) takes out the Hot Water, Vapor and Acid and pushes it into the Storage Chamber. This Chamber is keept hot by being Isolated like hell. No Blood flows on the Walls. Its just very good at keeping Heat in. This Chamber operates at a higher Pressure. Maybe many times higher than the outside one. A Simple Valve can be used to release the Mixture quiet fast.
If you dont want Acid in your Water, just change it for some other less deadly chemical. Works the same.
Now this is only one Path Evolution could arrive at. But it is cool :D
Sorry for the English
Ich bin Deutsch.