Heuvelmans system of classifying sea monsters based on his analysis of all known historical sightings at the time. Certain bits like the many-fitted and yellow belly have been removed from current classification schemes, but I find it all quite interesting.
https://cryptidarchives.fandom.com/wiki/Heuvelmans_system
His analysis was that there were probably giant 50ft eels, mutant seals with long necks, surviving zeuglodons, giant turtles, and some kind of marine saurian dinosaur throwback. Whatever the case, it seems like modern sightings of sea beasts has dropped off and might have something to do with marine pollution or changing temperatures