Since you imported the saved game, you may have to drain the water again using two irrigation machines. Have you encountered Sirrus outside the old memory chamber? If so, then you need to use the harvester. Close the door and lower it to force some water out of the basin. You should be able to go down the stairs to a control room and turn a valve to lower the water even further. Go back to the harvester controls and lower it again with the door closed, then open the door to let water in, close the door, and raise the harvester. You should now be able to reach the secret door.