When I started exploring Roslin Glen, the fish carving was one of the things I looked for first. However, as no-one seemed to know where it was, I came to the same conclusion as above – the face below Lover’s Leap.
However, the major problem with this theory is the fact that the face could not really be taken for a fish. It looks more like a monkey.
I’ve come to the decision that the fabled Fish carving is in fact one of the Hawthornden cliff carvings on the other side of the river, just past Wallace’s cave. The only representative carving there is clearly of a fish. See the Hawthornden page for details etc.