Of all the unimaginable animals of the past, the swimming ground sloth must be one of the strangest. Archaeologists have found bones that show the creature’s gradual adjustment to aquatic life.
Over the generations the sloth’s bones became denser, to stop it from bobbing up to the surface. Its limbs became more suited to swimming and its teeth adapted to a diet of sea grasses and algae.
The swimming ground sloth lived in Miocene period in South America. I will add more information as I find it.
(Source: Prehistoric Mammals by Donald R. Prothero)