Honestly what you are finding out is that the Physical surface node is a compromise between Firefly and Superfly, and it isn't really a PBR friendly node. If you want to learn PBR quickly, don't use the pre canned node that is a compromise between two render engines. In your fluid example above there is a far quicker way to get fluids that have refraction, specular, etc... Leave everything from Firefly somewhere else, because it no longer applies... Let me give you a quick and easy example.

Create an empty scene with no lights (delete all of them), set the scene to hidden line display so you can see what you are doing.

Go to the material room and set the background shader to look like this, make sure to check cast light... You will need a good HDR file to cast the light.

Poser has a few. Polyhaven has tons of them.

Add a prop and set the material to look like this (It is the glass shader that comes with P12 with the IOR changed to 1.33)

Set your render settings where caustics are turned on. Similar to this. Obviously pick whatever hardware you have, cpu, gpu.

If you did everything correctly, you should get something like this... You can not do this with the canned physical surface node that was for skin, and not much more....

This is literally PBR, without using anything PBR other than an HDR image.

I know that my sound odd, but it is. You can add textures to the water, etc, to make it look far different.

The best way to learn PBR, is to be able to wire it up yourself first... And more importantly have an environment for it to interact with...

I would start with any Blender tutorials on how to set up materials. Look for ones that are simple and ones that don't use nodes Poser excluded, like color ramps.

Just about any of the Cycles material tutorials will work.

---

Some things are easy to explain, other things are not........ <- Store ->   <-Freebies->