I was in Athen's Friday-Sunday and this blew up in the local Facebook group. Items reported there include:
It's not moving with the current and it took a curved path across the current.
It is a bull shark. A variety of shark that apparently can live in fresh water for extended periods.
It's some kind of motorized device.
Me... tough to say. Things in the water often aren't what they appear to be. Scaling the size is difficult on open water and current and wind make things behaviors look different. I recall when the jet crashed near the Rhinecliff Bridge. Everyone on shore kept saying the tail was sticking out of the water... that became gospel and I saw it reported by the press. Looking through my very good binoculars from under the bridge suggested to me it was twisted fuselage... as did cropping and zooming in on the photos I took. With a fairly significant delay the press finally reported official statements it was fuselage. Another case of eye witness testimony being false.
That said, I'm going with Nessie for now while waiting for verification.