A bunch of naked boys jump and splash in the water right on one of the canals of Venice. There's even the tail end of one of the gondolas to make sure the audience doesn't mistake where this is taking place.
It looks very natural, but if you look closely you'll see that none of them move out of the frame. That was probably the direction that WK.L. Dickson, or perhaps his cameraman gave them: watch the camera, and don't move to the left or right." Although, come to think of it, probably not. The directions would have been in Italian.