The Aeronautical Research Council published [Investigations of the Behaviour of Aircraft When Making a Forced Landing on Water (Ditching)] (http://naca.central.cranfield.ac.uk/reports/arc/rm/2917.pdf) in 1957 summarising research conducted before, during and after the war. It finds that the most important variable under the control of the pilot is the angle of descent, which should be as shallow as possible; without engine power the minimum glide angle of most aircraft is 4 degrees, and while it's possible to flatten out at the last moment a slow shallow powered glide offers more control.

The pilot's notes for the Spitfire Mk IX contain a section on ditching that reflect this: 61. (ii) (e): "The engine, if available, should be used to help make the touchdown in a taildown attitude at as low a forward speed as possible"

Note that, in the film, Collins pulls the stick back at the last second so the tail hits first; many single engine fighters like the Spitfire were prone to somersaulting or diving when ditching. He did miss section (a) of the ditching procedure, though: "The cockpit hood should be jettisoned."