[Ziggurat]

I had the same problem, expected him to be at lower level. He is on top cave area waiting for you. But if you already have done tasks for him at the upper level, such as the 5 flags, you may be at the underwater training. If you forgot to talk to him before diving and leaving the area, you won't see him again. The quest was to put the breathing apparatus on and go back to him to talk. You will be stuck with a quest in your log, but it turns out it is just bad bookkeeping. No reward that I can remember, so don't worry. Ignore it. Drove me crazy trying to solve a bugged quest.