First, let me say, I would have probably never done this movie without this location.
After doing some research on Internet, I found Oliver Wegner's web site: Rambo town. Oliver is also a 'First blood' huge fan and he went visiting multiple times Hope and the surrounding filming locations. But he reported there were "no official records of the filming crew, this location seems to be missing. Actually nobody still today remembers the lake. There are many many big lakes in British Columbia but anyway we cut those from the list that didn't come in question.. ".
Well, it was not really optimistic but I was not ready to give up so soon ;-P.
In order to find the spot, I watched closely at this opening scene. You can notice there are 2 prominent and most recognizable mountains: Golden Ears peak and the Mount Baker (see contrasted picture below).
In fact, this is not a lake. This is the lower Lake Pitt River.
Then these 2 mountains gave me an orientation. And with the help of Google Earth tool and some patience, I could match the topography and isolate the spot.
Following are some capture shots (orientation, topography and aerial view) taken from Google Earth. Fortunately, this area was in high resolution else it would have not been possible to discover the spot.