It is the misfortune of this waterlogged British indie to be a true-life drama about a female endurance swimmer, coming out when people are still talking about Annette Bening in the Hollywood underdog heartwarmer Nyad, also about a female endurance swimmer. Nyad cuts powerfully and confidently through the water. This film, sadly, seems as if it’s got cramp after picking up a dozen verrucas in the footbaths.

Kirsten Callaghan plays Mercedes Gleitze, the young Brighton swimmer of German parentage in the 1920s on a mission to become the first British woman to swim the Channel, but facing snobbery, sexism and xenophobia at every stroke. Her main competitor in the female Channel-swimming stakes is the haughty Edith Gade (Victoria Summer), a notorious cheat – based on the discredited swimmer Mona McLennan – whose scandalous fakery contaminates Mercedes’s honest efforts and forces her to repeat her Channel crossing with a “vindication swim” when the weather was too rough for the attempt.

Edith’s thin, snippy rivalry with Mercedes over cocktails is the closest this film gets to actually coming to life. Our heroine naturally has the standard-issue grumpy trainer, Harold Best (John Locke), apparently a fictionalised composite partly based on Bill Burgess, the former Channel swimmer who coached the American swimmer Gertrude Ederle.

All this could have made for an intriguing, rousing and unexpectedly complex film. But it sinks. This is somewhat due to the pasteboard-thin period detail and genre cliches (George Clooney’s The Boys in the Boat was holed beneath the waterline in the same way). But it’s also due to some very uncertain line readings and torpid direction. This movie is wearing water wings made of lead.

• Vindication Swim is in UK cinemas on 8 March