With minor exceptions, you guys are hopeless in sticking to a topic and solving puzzles, but we have at least attracted the poet deploreate of Pnet. Welcome.
My criteria can be met by all the meltwater rivers of Antarctica not just the Onyx, which happens to be the longest.
But let's begin logically and with minimal knowledge. It's easy to avoid sharks and stingrays if you avoid the ocean. Exception: bull sharks have a unique physiology that allows them to adapt very quickly to fresh water, thus allowing them to be found very rarely up rivers. So, if we want to worry about bull sharks, we have to stay above rapids or waterfalls that they can't traverse.
Stingrays aren't found in arctic waters, and sharks are just about everywhere but apparently not around Antarctica. Therefore, my criteria can be met even when paddling the coastal ocean around Antarctica. That's 11,000 miles of coastline. Where's Verlen Kruger when I need him for a tandem partner? No bears, lizards or snakes on that continent.
I've always hoped to paddle the Jordan River from the Sea of Galilee to the Dead Sea -- along which, somewhere, Jesus was baptized by John the Baptist -- but I am blocked by at least one poisonous viper. The same holds for all of England and Scotland, which would be ideal except for one poisonous adder. For 32 years I've wanted to "loch drop" diagonally across Scotland, land of my Clan Cameron, but that's another dream that will die with me.
But, ahah, I have all the lakes and freshwater rivers of Ireland to paddle in with no worries about the Six Satanic Species, aside from the pesky Bull shark, which I can avoid. In fact, Ireland has no snakes at all. No wolves either, as the Irish exterminated them as did the English and Scots.
If I want warmer paddling I have all the fresh water of Hawaii. While there are occasional reports of pet alligators escaping, they are not native to Hawaii nor are crocs, bears or poisonous snakes. The problem is that the Hawaiian islands don't have a whole lot of fresh water lakes and rivers.
Therefore, my ideal warmer weather spot after I finish with Ireland are the freshwater lakes and rivers of New Zealand. Not only are the Six Satanic species absent from all of New Zealand, there is virtually nothing there that can even bite a human. No wolves, foxes or even ants, and only two varieties of innocuous spiders. And no place has more spectacular paddling scenery.
I'm always interested in learning about new paddling venues, and this was undertaken with just a few minutes of research. I'm sure there are other spots on the globe that meet my criteria. But Canotrouge is ultimately right: I am likely to paddle these dream spots only in my head.