The Hand of Satan in the Sea of Darkness, Legends and Superstitions of the Sea and of Sailors in All Lands and at All Times (1885) by Fletcher S. Bassett. 

Olaus Magnus 1539, "lobster dangers".

An ichthyocentaur (parts human, horse and fish) plays a viol on a map of Scandinavia from the 1573 edition of Ortelius’s map. In the upper left hand corner is the phantom island St. Brandain.

Sea Lore of the Atlantic

For many of the early civilizations, the Atlantic was part of a vast unknown ocean encircling the world, a sea associated with chaos and death. The Arabs called it The Sea of Gloom or The Sea of Darkness and depicted it with the Hand of Satan rising up from its waters. The 11th century, geographer al-Biruni wrote of it, “here there are many ways of losing oneself.” Many considered it unnavigable with its rough weather and windless stretches, patches of dense fog, seaweed, ice and slush, and then there were the monsters swimming in its waters. There was a long-held theory, going back to at least the first century with Pliny the Elder’s Natural History, that every land animal has an equivalent in the ocean. There were thought to be sea dogs, sea cows, sea pigs, etc.. Some of these are now the names of real animals—sea lions, sea horses, sea cucumbers. 

Sea pig. Olaus Magnus's, 1539 Carta Marina

Sebastian Munster's Chart of Sea Monsters, 1552

Of course, some of these monsters were real.