id tech 3 shaders make water like this possible in a quake engine.
the water shown here is simply additive blended textures that have either frame animation (surface, caustics) or multiple scrolling planes (waterfalls) which are lit (lightmap or vertex) and have soft, alpha blended edges. the caustics are just decals, and the water color is faked by tinting the world texture beneath the surface.
the water assets account for around 4% of the game's current 70mb download size.