The dam wall pretty much faces a westerly direction so we can assume this location is somewhere NW. Next we can see the hillsides behind some of the closeups of Whang in Sik. Anyone who is familiar with Sai Kung's ridgelines will perhaps recognise some familiar shapes here in the form of Pyramid Hill and Bendy Hill (Wan/Kau Kuk Shan).
Bendy Hill and (just) Ma On Shan to the right
Speaking of Ma On Shan, here is a shot of it from the angle of Sai Kung. Of course this gives us a fairly decent triangulation of where this scene was filmed. However, since I haven't walked up this particular hill (yet) I can only give an approximation of where it is.
The scene of course ends with JC turning superhuman thanks to the wonderful anaesthetic powers of bong water.