Were they purely decorative or did they serve as warnings to sailors?
Did the cartographers belive that specific monsters roamed specific parts of the sea?
Example link: https://espliego.files.wordpress.com/2008/01/1500px-carta_marina.jpeg
Oooh, ooh, I asked a similar question a while ago and I think you'll get the same kick as I did from /u/sunagainstgold answering it. This is obviously not discouraging any answers coming in, but that post needs to be read.
