Dive into the waterway go down as far as you can go and then go towards the BACK of the castle (I think it's west). There is a lever on the west(?) wall. It may be hidden by some plants.

This sounds stupid even to me, But how the hell do I get it that freaken castle. I found the waterway that runs underneath the side, but it's blocked by a grate before you can surface inside. Is there a lever I'm missing or something.. I'm trying to lower the drawbrige for the Ninja quest, but I can't get in..

Thanks