This book was ok. Not bad, but not especially good either. My main problem with this story was, that I never felt that any of the characters were in any danger. This was disappointing, because the main part of the story is that they're supposed to be in great danger. The Doctor and the scientific team are trappend in an underwater base, that is under attack from giant sea creatures and troops sent by the Darksmiths. Children may find this story much more dramatic.
The best part was when the identity of Gisella was revealed. It's soon very clear that she knows much more than she says, but I wasn't expecting her to be who she turned out to be. I can't say much more about her without spoiling the whole thing though.