Meet the deathclaw's bigger, tougher cousin. These monstrosities hang out in large bodies of water and ambush nearby prey. And to them, everyone and everything is prey.
The pre-war researchers and geneticists wanted another biological weapon, one that could both charge enemy armor head on and camouflage itself behind their lines, quietly mobile in its waterways and rivers. A predator that could be deployed and would independently breed into a huge threat with no outside aid. When fully grown, they are almost impervious to small arms and can flip over even small military vehicles with pure strength. Most frightening of all, the original desired trait of the Jackson's chameleon that deathclaw scientists were attempting to replicate, skin color changing, is functional in the crocodilian variants. This combined with their patient ambush-mindset would have made any enemies wary of attempting to hunt them down in the waterways.
However, the project didn't go as planned. The males were so intolerant of one another that they refused to hunt or breed properly anywhere but their own established territory, which limits their effectiveness as their numbers would never swell in one area enough to properly disrupt local enemy outposts. Their aggression, supposedly made more extreme due to genetic tampering, was not reliable enough to compel them to storm groups of enemy infantry when they weren't hungry. Worse still, many of the alphas were showing signs of higher than expected intelligence, which greatly complicated the precautions needed to breed, house and deploy them, as well as making their behavior even less predictable when released. The project was deemed unfit for military use, and the specimens already created were not properly disposed of before the Great War.
After the world ended, they could be found making their way slowly towards large waterways searching for new domains. Their low breeding rate has kept them in check for the most part, but wasteland travelers with especially sharp perception might see those dark green eyes break the surface of the river or lake to survey the land. Eyes that look back at you with a cold intelligence and a knowing stare. It has a hunger kept in check by a patient predator's instinct. It sees you, and it smells you.
It knows you need to drink sometime.
I might have combined inspirations from both Fallout and Jurassic Park for this one