This is known since a long time, but we now have a proof it worked before, at least in 2012:

Here the depicted map is Citadel, but it likely required a modified .shader in a modified .pk3.

We have a map to investigate this:

• https://github.com/UnvanquishedAssets/UnvanquishedTestAssets/tree/master/src/map-test-water_src.dpkdir

Here is a related talk to improve the material syntax, but improving it doesn't make sense if the shader doesn't work to begin with:

• talk: liquid shader syntax design #261