It could have been a salt water species, but it was in the Ocala National Forest, 40 miles from the ocean, and the river is entirely spring fed. The ray was only a couple hundred yards from the spring.
That is odd. The only purely freshwater species I can find are in SE Asia, Australia and now South America. It may be a previously unknown species or someone dumped a specimen from overseas in the river, or it may be a saltwater species that wandered upriver and became acclimated to fresh water. They are quite common here in Choctawhatchee Bay which at times is more fresh than salt after heavy rains upstream in the north....