Okay, so after my hard disk broke down and I also had to endure a Windows update, I’m back running 
In order to get this setup to work, you need to understand the Phoenix cache channel types.
So you have a liquid simulation in the simulator called “Water”.
When you go to the Simulation rollout, you can see what channels are simulated in the loaded cache for the current timeline frame. For example at frame 80 I have:
[GRID: 1 960 000 cells, 140x140x100]
- Liquid/Temperature (0.00 : 1.00)
- RGB
- Velocity (0.00 : 413.57)
[PARTICLES: 22 674 total]
- Liquid: 22 674 (Pos,Vel,Age,RGB,ID)
- Foam: 0
- Splashes: 0
- Mist: 0
- WetMap: 0
You have 3 grid channels and 5 particle systems. The grid channels are Velocity, RGB, and Liquid which is in the same channel as the fire/smoke’s Temperature.
So you have a second simulator for rendering the liquid data, called “Smoke”.
You load the data from the liquid simulator - you can do that in two ways:
1. Either directly browse to the caches of “Water” from the Input rollout.
2. Or use the magic macro for the Input path:
$(same_as_output) Water
^ this will be a more portable way to reuse caches and will work even if you copy the scene to a different directory later.
When a simulator has loaded cache data, all other simulators see it as a mesh. We have prepared a docs page explaining all possible ways for simulators to interact with one another: Phoenix for 3ds Max
Since we don’t want the simulation of the “Water” sim to interact with the loaded caches of “Smoke”, we exclude “Smoke” from the Scene Interaction rollout of “Water”.
Once the cache are loaded in the “Smoke” simulator, we want to render them as a volume.
We don’t want any Fire, so under the Volumetric Options for Fire, we set Based On to Disabled.
Now we can map the smoke color and the smoke opacity to the data from the “Water” simulator.
In the caches we have 3 channels, so we should choose which one to use for the smoke density/opacity. RGB will be weird if we use it as density, Velocity/Speed will actually be quite cool, but the simplest way would be to get the liquid density from the Liquid/Temperature channel.
This is why under Smoke Opacity, you should pick Temperature.
Now you have to be aware of the channel ranges and adjust the opacity curve. Right now it goes from 0 to 2000 because by default it’s ready to shade the Fire/Smoke simulation’s temperature in Kelvins which goes up to 2000 or more.
However, in this case you have the liquid density, which goes from 0 to 1 - you can check this in the Simulation rollout’s cache file content box, which I also pasted above.
You can also check the range in the opacity curve - it’s marked a a teal/green zone. In this it would be very thin.
What you can do for a start would be to edit the 2 points of the curve. Set the lower left one to X/Y = 0/0, and the top right one to 0/1.
Render it out - the “Smoke” simulator will now render a blue volume where the liquid is. Since you’ve mapped the Smoke Color to the Smoke channel, but such channel does not exist in the cache, it’s data will be read as zeroes. The color gradient maps the data of 0 to a blue color.
You could experiment and map the color or the opacity to the Temperature/Liquid channel, or to the Speed channel which is the length of the Velocity channel, or you can also keep the color constant.
Hope this helps clarify what is going on in this setup 
Cheers!
