The most famous of the Sloths, we all know more or less, is the Megatherium. A ground sloth sturdy-looking, with more than 3000 kg (6.600 lb) . But there is another Sloths, thalassocnus, which has the peculiarity of being prehistoric and water.
The thalassocnus moved slowly and peaceful, sometimes snorkelling hunting dangerous algae and mosses with bladed teeth.
Like the Megatherium, the thalassocnus also had big, strong arms, but not to grab the branches of trees, but to cling to the seafloor, holding well to the underwater rocks while feeding.
This Sloths was not as big or heavy as its cousin the prehistoric ground sloth, in fact, could be bigger for being aquatic as it can better withstand the weight ... but no. The thalassocnus could measure about 2 meters long (40 ft.), and weighs 120 kg (264 lb.). Sometimes he could leave the water and feed on land plants, but not a large as its cousin, it could easily fall prey to predators such as Sparassodonta or Amphicyon.
Anyway, you do not think that it is rare to see a sloth in the water. Sometimes, amazon overflows, and the only way to move from tree to tree is swimming. Here you can see a sloths, swimming ... it does very well.
It existed between the Miocene and Pliocene (between 25,000 and 2.5 million years ago) in what is now known as South America.