To try to clarify it all:
The loc given by PoeticJ is SPOT ON. If you paste it into one of your maps, you will see that it is outside the four large pools displayed on the map. That is because he isn't in one of those!!!
DO NOT follow the underground pool system. You will not get there.
You must go to the top of the pyramid (reached by using the caves on east side of courtyard).
I was able to see his hand waving from the "plank" at the top of the pyramid and leved down to him. If you are able to land on the edge of the tank, I assume you could make your way back out without gating. Once in a tank however some form of gate is necessary.