Don't go back in the water just yet...

[bunrab]

This one is an ocellated, or ocellate, river ray, Potamotrygon motoro - it has several other common names besides ocellated, which word refers to the two-tone spots with the darker rim and paler center. Because of the many common names, it's easier to just refer to it as a Motoro stingray. It's another one of the freshwater ones from the Amazon (hence the Potamo- in its genus), and the National Aquarium has one, which is larger than average, about a foot and a half across.

Here's a really nice page from someone who has a pet Motoro:
http://www.dcpaleo.org/Leisure/Animals/FishesStingray.html