8. Water

First, you have to create a new room to fill with water at the end of the last corridor. Switch to Template Mode.

In the Template control panel, choose a Hollow cube, then create the room as shown below.

Don't forget to texture it like the corridor. But you can choose other textures if you want.
Switch to Select Mode,

select the cut brush you used to connect the second room and the corridor. Clone that entrance to connect the corridor to the new room, as shown above. Then, select the new room and scale down the floor.
Now switch back to Template Mode, and in the Template control panel select a solid brush. Place it as shown below and press the Enter key.

Select the new brush and apply a water texture. Click on the Face/Brush Attributes button.

Set the brush to Empty, Wavy and Water as shown below.

Press the PageDown key in order to switch to Face Mode. Then, click again on the Face/Brush Attributes button.
In the Face Attributes dialog, set the Transparent option and its Transparency Value to 100.

Switch back to Brush Adjustment Mode by pressing the Page Down key. Switch to Template Mode. In the Template control panel, select a Light entity, place it in the center of the room next to the ceiling and set its Light value to 500 in the Entity Dialog (see the chapter 3. Lighting for more details).
Then Compile and Preview your level.

Here's a view from the corridor where you can see water reflecting in the mirror on the corridor.

From inside the room you will see that the gravity is modified in the water. You also can see the floor through the transparent water.