Slider of some kind for sure. I would say location information could help narrow it down- but seeing as red eared sliders have been introduced so many places... that may not help any.
Cooters do not have the serrations on the back of their carapace. The back edge of cooter shells is smooth. There are other differences as well- but this is the one I noticed first in your photos. The patterns on cooter shells is also typically more intricate that sliders, but the shells on most older aquatic turtles just tend to look black with no decernable patterns.