You ask what animal in the Americas would fill the niche? Not hippos. Or anything as agressive. More like a Dugong or Manatee-like creature. They are day eaters, but they fit into the aquatic grazer mold.
See this on where they came from, where they went, and how they have changed (early models had legs). They first developed during the Eocene, and have been around ever since in various iterations. Anywhere the water is warm and there's aquatic stuff to munch. They can swim the ocean, and do, but prefer to swim up rivers where they can.
They would have had time to develop by the pleistocene. Here's the wiki on Sirenian evolution.
Apparently at various times there were multiple models of sea cows swimming around, filling various niches. And there were a lot of them during the Pleistocene:
Pleistocene-age manatee bones have been excavated from over 24 sites in Florida, and men hunted them as soon as Indians arrived in the region. link to source
Here's a link to a scientific paper regarding the various kinds around the world. Notice that the ones in Florida that were found, were linked to a period just before what you are looking for.
See this handy link for the current ranges of the species in South America and some of the fossil records of it for the area. It's pretty extensive and likely that they were there, by the time period you are seeking.
An earlier version might have been less docile, and there's plenty of extinct versions to choose from.
Anyway. it's a place to start and they do, at least somewhat, fit what you are looking. Only a hippo is a hippo. As far as I can find, a specifically night feeding, aggressive aquatic feeder is a pretty specific niche--hard to find something exactly the same that isn't a hippo--but some of the early evolution pictures of the sea cow look a bit hippoish, from before they lost the legs. Problematically, by the era you indicate they would have lost the legs again, but maybe you can tinker with that--evolutionarily they've been in and out of the water a bit...
Quite a bit smaller, but still reaching into this niche: the capybara. During this time and in this place, they reached about 100 pounds. Not huge but still interesting.
Capybaras graze in meadows, bed down in the woods, and spend much of their time in water. They require habitat that offers all 3 of these environments. They use water to escape from predators and to cool down during the heat of the day. They are not as helpless as they look for they are fast runners and swimmers, and they have thick hides. Nevertheless, jaguars were likely their most dangerous enemy in North America during the Pleistocene. The extinction of capybaras here probably contributed to the extirpation of jaguars in this region. link to source
Here's a modern photo of these guys, which are about 1/4 the size they would be in the Pleistocene.