Shallow Lake Waters                                    -      N      - 
(-------------------------------------------------)     W <---(M)---> E
                                                        -      S      -

You are swimming through the waters of an enormous subterranean lake. 
This section is just shallow enough for you to stand on the bottom, but it
is much faster to swim rather than wade along.  You are unable to see the
shoreline through the darkness, and it is also impossible to tell what lies
beneath the surface of the water.  You doubt that anything dangerous could
survive this far underground, but you keep your eyes open just in case.