I can't get to the back of the castle in order to enter. I have played with the lever at the bottom of the moat on the SE side but all that happens is the grate covering my exit is removed. Seems to me that I need to get the west sige grates open in order to get to the rear of the castle, but I have no idea of how to open them. What am I missing?

It sounds like you are in the right area - EAST side. Where the open grate is swim down then swim south. There is a lever at the end of this underwater tunnel. Click the lever and the grate directly above you will open. Swim up and you will be on the back side of the castle!