I have killed Scabban, flipped the lever, jumped into the water and swam out of the room. I can only swim down the tunnel in a southwest/west direction and end up drowning--many, many times. I read where I need to swim southeast into the main channel then go NE-E-SE into a small channel to find the underwater lever and get the gold key. Cannot find a way to go NE. What am I doing wrong?

Of course, you want to surface as soon as possible after you jump into the water and swim through the open round grate and on through the tunnel under water. It is not that far, but took me a few tries to make it through the underwater tunnel with drowning. After you surface, then take your time swimming around on top of the water and looking around - it is huge back in there and all of it is water, except when you go inside the tombs (use your map), but you don't have to stay under the water. It is a big circle in there with the tombs of the heros and also two tunnels that lead to the gratings where the rat pie guy is. When you find these, you will find the gold key lying on your side of the underwater grating now; and there will be levers to open both grates on each side that will let you out into the familar area where the rat pie guy is. Hope this helps you out!