Following the battle with Scabben, I've swam into the area with four crypts in a big square surrounded by water. I try to open the locks with my rogue but get the message that something is missing. I am completely out of ideas. What do I need to open these doors??

Thanks for the help!

You remember those keys that the stout brother lost...Swim around that big circle, there is a passage that leads to the other side of that grate you saw earlier and swim underneath the water and grab the gold key it opens all those doors. Just explore underwater it pretty easy to spot once you know what your looking for =).