This is another quick walkthrough showing you how to get to the All Knowing Vortigaunt in Half Life 2. The stage is 'd1_canals_13', - last level of Water Hazard. Very easy to find once you know where it is. Make sure you have quite a bit of health though, as the radioactive waste tunnel before the cave is quite long. There is plenty of health near the vortigaunt, so you should be able to get out easily.
EDIT: This is now an achievement on the PC version of HL2!