Is this a photo of Alfred Hitchcock floating on the river Thames, 1960s?

That is not Alfred Hitchcock, but a mannequin (a dummy) that looks like him. It was used in a trailer for the movie Frenzy (1972).

You can watch the trailer on YouTube:

The head of the dummy can be seen in the next photograph of Alma Reville at her Bel Air home. Photograph by Philippe Halsman, 1974 via Magnum Photos.