This book has no science.
The content is a bit dark with somewhat graphic descriptions of violence. I cringed at one point and came close to cringing a few other times, and that usually doesn't happen to me when reading vintage series books. This book almost fits into the modern young adult book category.
This book is a travelogue book, and I did find some of it to be more than I would have liked. However, most of the descriptions of Holland were interesting, and I enjoyed the travelogue aspect more than I do in the average Nancy Drew travelogue. I learned several things about Holland that I did not know.
The character of Gretchen is rather interesting. Take this passage, for instance.
Gretchen shrugged. "Oh, I knew you could take him, even with your hands tied. He is too slow and stupid to get out of his own way. And I was right, wasn't I?"Gretchen is dead serious. She makes more bloodthirsty comments in other passages. She's a bit scary, just like much of what happens in this book.
Scotty smiled weakly. "I wish I'd had your confidence."
"If he had killed you, I would have shot him," Gretchen assured him earnestly.
Scotty stared. "That's nice," he managed.
This is a thrilling book, with a suspenseful climax. I greatly enjoyed it.
I had the same feeling while reading the first two-thirds of this book as I had while reading The Wailing Octopus. This may have more to do with the plots being centered around scuba diving than anything else, but I felt tense expectation while reading this book. There is something about diving down in the ocean that adds suspense and makes the reader expect complications. Rick and his friends have some very interesting and dangerous complications while scuba diving!
Unfortunately, I did not find the last one-third of the book to be as interesting. The suspense disappeared, and the plot centered around extremely detailed technical information about deep-sea diving. The information was too much for me, and I had to begin skimming it. The climax of the book wasn't that interesting to me, either, and I skimmed the last few pages of the book.
The first two-thirds of this book is excellent, while I found the last one-third to be mediocre.