Getting this book today brought back good memories of when I first became interested in cryptozoology. the book that got me interested was Jeanne Bendick's The Mystery of the Loch Ness Monster, but Sea Monsters sat right beside it in the library, so I would always check it out, too.
Here's a book description:
"Do sea monsters exist? Based upon the survival of the coelacanth, a species of fish 390 million years old, and upon knowledge of biological mutations caused by ecological changes, the author feels certain that a wide variety of sea monsters do exist. During the course of his extensive investigations, James B. Sweeney researched ships' logs, naval archives and newspapers around the world, and conducted hundreds of interviews with eyewitnesses of monster sightings.
"Strange as it may seem, scientists know more about the surface of the moon than they do about the depths of the sea. Perhaps during the next few decades they will be able to expand their exploration and knowledge of the ocean floor and its inhabitants. Meanwhile, here is all that is known about sea monsters, gathered from reports as ancient as 700 B.C. and as recent as the 1970s. They include accounts of numerous grotesque beasts ranging from twenty-ton jellyfish to the elusive Loch Ness Monster."
You can buy the book on Amazon.