How to get the Rosetta Stone in Ultimecia Castle - FF8 Guide
In order to get the Floodgate Key you'll want to put all three men or two men into the same group and two females into the other group. You can use the green circles to switch between groups as well as switch group members (if both groups are standing at the same green circle).
What you're going to want to do is place the group with the two or more females on the left elevator (as pictured above). Then switch to the other group (with two or three males) and move them onto the opposite elevator. Switch back to the first group and grab the Floodgate Key, then you can step off the elevator with both teams. For a video explanation of this puzzle check out my How to get the Floodgate Key Youtube video.
Once you obtain the Floodgate Key you will want to head over to the dungeon area (pictured below) which is where you will find the lever that you need to unlock with the Floodgate Key then press. This will stop the water flow in the part of the dungeon with the Rosetta Stone.
Return to the entrance of the dungeon and go north through the chandelier room. On the other side of the courtyard you'll find the church and in the back of that church an organ (pictured below). What you need to do here is press all four buttons at the same time until the organ stops making noise then press Start to back out.
If you did this right then all the bars will be lowered in the waterway which is found by exiting the church and taking the northeast route. It's a linear path to the waterway from here, once in the waterway go to the right and that's where you'll find the Rosetta Stone (pictured below).
Note: Just south of where you find the Rosetta Stone you'll find a door which leads to the armory (you actually unlock it by going through this way) and it makes a nice short cut through the dungeon if you can remember it.