i played it many month ago, so i'm not sure...
If the 4 entrances (corresponding whith 4 areas) are numbered from left to right,
After you active the switch in the area n 4, return to the second entrance
there is a big room which is now plenty of water, so you can access the hole at the other side, you will find the hiding area.
Remember i'm not sure of the area order and number, try... and good luck!