Maps and Sea Monsters

When looking into 16th century maps of the world there seems to be a lot of weird sea monsters dotted around the place. One reason for this could be that it is like a diary of where the crew have been and where they saw each monster. Another reason could be that when they were making the map they put the monsters where they hadn't explored and was unaware of what was there, therefore the monsters were a symbol for the void of the world.