I can definitely say this is a VERY OLD MALE MELANISTIC slider or cooter. I would need to see the side of the head to know for sure. Some older slider males (mainly red eared sliders) will lose their patterns and gradually darken with age. What state was it caught in??
No matter what type of turtle it is, that turtle is aquatic and needs to be put in water asap. Is it possible to return the turtle back to the pond? It should go back. What part of Florida, if you don't mind?