The impression I get is that the game struggles with rendering the water texture, meaning that surface area is the biggest concern... but I'm not entirely sure on this, which is why I'm asking :rolleyes:

just taking punt, but i'd say it's the surface area that is more costly. Because it's just the surface that does the reflection/refraction, and large surface areas seen at once is what hits the performance.

I can't imagine how depth or volume of water requires too much rendering/calculating.

Basically, it's the large surface areas of water seen at once that is the problem, not the volume.

Imho, you have the answer in the way one should make a water brush: 5 nodrawed faces, the top one with water.

As such there is never a notion of depth (can't swim, dive). If Source handles collision with a water face the way some other engines do, it's all about the surface, and the rest is just non existant in game.

I haven't found water to be that much of a performance issue really, and I'm making a map with alot of water in it. But if you don't use the right optimizing on models, fog etc then yes it will start getting out of control.

Also, for the mappers that want to use water to surround their maps: DO IT IN THE SKYBOX.

I've seen many maps with water that wasn't made in the skybox and was actually huuuuge brushes of expensive water all around the level, it lagged horribly.