While the Inquirer isn't always the most reliable source in the internet, it appears they have confirmed that the same texture pack was purchased by Valve Software and GSC Game World from Marlin Studios.

"Thanks to reader Rob S. for pointing out that the water textures used by both Valve and GSC GameWorld, the makers of STALKER, come from a texture pack sold by Marlin Studios. The chaps at Marlin make all kinds of exciting textures, including 'Home & Office Furniture - High Detail, Low Price!' and 'Traffic - Low Poly Pretextured Vehicles, SUVs and trucks you can even animate!'

Stunning stuff.

Valve has confirmed that it picked up some textures from Marlin for HL2, and separate sources have discovered that the IMP_Light texture that it was thought could have been pilfered from Doom 3 was, in fact, just a standard Impact Light texture, rather than referring to the Doom 3 monster."

Thanks to the Inquirer for clearing the issue up.