(Follow-up post)

1. Create the level or area where you want the water (i.e., a room with a pool)
2. Create a rectangular 3D (not 2D plane!) brush that covers the surface like the water surface would, and is not tall (2, 4 or 8 units high, as it must fake being a 2D surface. I used 2, the least, the better).
3. Assign any water or liquid texture to it, and CSG add it to the world.
4. Edit the upper face properties and activate "mirror", "unlit" and "translucent".
5. Edit the bottom face and set it just "translucent".
6. Set the brush non-solid.
7. Create a 2D plane as wide and long as the "fake" water 3D brush. Place it inmediately above, but not overlapping, the upper "fake" brush surface. Make it zone portal (with the same menu you would use to add a water 2D surface, Add Special).
8. Select the upper face and make it "invisible".
9. Add a water entity inside the pool now delimited by that portal.

     
That's it. Basically you just created a normal water zone, only that you turned the 2D surface invisible, and replaced it with a fake 3D water "surface" that, being a 3D brush, lets us get reflection by altering the faces' properties.







Hint: with this method, you can manually divide the water surface in areas that reflect and areas that not. Just put the fake 3D surfaces in areas where it is needed, and use normal 2D water planes for the rest. In rectangular maps, this is very easy. It is a cool effect for a flooded cave or so: water inside the cave would reflect, water outside in open sea would not.

Sadly this method has one limitation: you won't be able to see underwater from the outside (but you can do the opposite).





It's a bit silly, but I hope you find some use for it. Maybe there is some way to improve it.

You are right, it does! I think this mistake has something to do with the textures and lights I used during my first tests. Thanks, removed the step/limitation from the original post.