Maybe I'm wrong, but I think what Urge is refering to are the prehistoric animals that lived in the oceans, rivers and even lakes.
Here are some examples:
A prehistoric shark (a picture from Disc)
A leedsichthys:
This guy still hangs out in some of the deeper and darker parts of our oceans:
The black devil angler fish (ugly little bastards, as are all angler fish, but interesting)
You've all heard about the giant squid, right? Well they actually exist. These babies can get up to 150 to 200 feet long. I was watching a show on the Bermuda Triangle (Discovery of Natl. Geo can't remember) and one scientist (not cryptozoologist) said that it may be possible that some of the boat disapearances were due to one of these animals (possibly by accident on the animal's part). That area of water is extremely deep. We're not 100% sure how deep it really is, and we sure as hell don't know all of the life that exists in those depths, and that's true for most of the oceans. You know, the deepest part of the Mid Atlantic Trench could be as deep as Everest is high. Some scientest will tell you that we know more about outer space than we do about our own oceans. Pretty cool.