Quote
http://www.kiya1.de/beyond/act1maps/waterriddle0.jpg

Nope:
"6 is thrown" are the 6 levers in the 6 rooms.
"One not shown" => is the 7th lever in the pit - and this opened the door.
Kiya

Careful with that screenshot. My levers were in a different order: 4, 5, 6, 1, 2, 3 as marked on your map.