Masking out land in Landsat image using either ILWIS or Grass

Hello:

I have been trying for some time to mask out the land area around one lake
in a Landsat image of a bunch of lakes. I want to know how I can just
isolate the lake and make the land disappear and not be included in the
analysis. If anyone can help me I would really appreciate it. I have only
been able to make submaps and that is not good enough since it still
includes some land and other lake components in the submap. I have yet to
be able to create a polygon around the lake and clip just the lake area out
of the Landsat image.

I am very new to grass, but I am somewhat familiar with ILWIS.