Not Photoshop but good, old, traditional blue screen stunt work with a mock-up aircraft (the movie pre-dated Photoshop by almost a quarter of a century...which probably makes most of us feel REALLY old!).
The great thing about this forum is it makes you look, read and dig around. I haven't seen the film since it first came out but do vaguely remember the "walking on water" scene. But thanks to the net I didn't realise the Billing Biplane was seen in two guises - German and Japanese...we're seeing the Germans getting dunked. The Eardley Billing Tractor Biplane replica flown by David Watson appeared in two guises, in more or less authentic form, impersonating an early German tractor biplane, and also as the Japanese pilot's mount, modified with boxkite-like side curtains over the interplane struts, a covered fuselage, and colourful "oriental" decorations. https://en.wikipedia.org/wiki/Those_Magnificent_Men_in_their_Flying_Machines#cite_note-30 Eardley Billings | Aircraft |