in single player you dont see yourself in 3rd person so they whouldn't have bothered to add it (when they made the game I dought they knew how popular multiplayer whould be)Ravendust said:I don't think the HL engine can support it, even though it is quite simple.
My reckoning is, there are quite a few instances in HL (single player) where this could've been used if it were possible on the engine, but Valve didn't, so I doubt it was possible. You can do it on HL2 though.
If you want to try modifying the engine, you'd need when a player entitiy collides with a !TEXTURE (liquid) to play a .wav and create the water spray sprite.