As of yet, only 2% of the ocean has been explored. And last year alone, over 13,000 previously undiscoverd new species were discovered. So what does one call an undiscovered species?
In 1892 Dr. Anthonid Cornelis Oudemans, director of the Dutch Royal Zoological Gardens at the Hague, published his definitive work on cryptozoology – long before cryptozoology was even a popular idea. Titled The Great Sea-Serpent, this comprehensive work not only describes some 150 sightings (dating back to the 16th century) but also presents various hoaxes and alternative theories.
Oudemans dared to name the Sea-serpent: Megophias megophias. He concluded that the infamous cryptid was something very much like an elongated seal. For this and his other varied conclusions, the reception of the volume was "respectful but cold."
But you can judge it for yourself... a PDF of Oudemans' The Great Sea-Serpent (illustrations and all) has been online for some time. Have fun!
Oudemans may have written the book on sea-serpents but it's Bernard Heuvelmans who is broadly recognized to have been the father of cryptozoology. In 1958 he wrote his ground-breaking volume entitled On the Track of Unknown Animals. But I'm much more interested in his 1968 volume, In the Wake of Sea-Serpents. Because I like the ocean. It's blue. And mysterious. And I like the idea of undiscovered creatures in the ocean. And the book is fun to read (or browse through).