The title of this invigorating documentary about open-water swimming seems at first to be a wry note-to-self regarding something competitors essentially have no control over: the possibility of becoming shark food. But, as practised by Australian waterman Mark Sowerby, it turns out to a surprisingly deep and empowering maxim about choosing to accept apprehensions and fears, and not being picked off by one’s inner vulnerabilities.

Sowerby is that oft-spotted species: the investment banker seeking redemption. Adrift among the 1%, he pivots to long-distance swimming and makes a traumatic crossing of the English Channel in 2015. Then his company becomes chum for short-sellers. His self-esteem in tatters, depression swallows him up. Realising he can process the trauma with intensive pool time, Sowerby decides that completing the other six stages of the “Oceans Seven” – a set of brutal channel crossings around the globe – is the tonic he needs.

With the challenge designed to make participants confront their aquatic “kryptonite” at least twice, the physical side is daunting. Hyper-dynamic drone shots underline Sowerby’s plankton-like progress. One mordant doctor sums up the effects of progressive hypothermia – enemy no 1 in the Scottish-Irish North Channel (34.5km) and Japan’s Tsugaru Strait (19.5km) – as “everything slowly getting worse – a very miserable situation”. Great whites aren’t even the worst thing about Hawaii’s Moloka’i Channel (42km); that would be the nightmarish-looking cookiecutter sharks that rocket vertically up from the depths to take fist-sized lumps out of flesh.

But more than these aspects, marathon swimming is an exercise in confronting one’s own mind and developing a certain relish for it. Sowerby’s Gethsemane comes during a nocturnal crossing of the Catalina Channel (32.3km), where he confesses to hacking for hours through a soup of self-loathing. Director Jeff Tseng puts insightful emphasis on the coach’s role in deciding whether this torture should continue or end; the man on the boat here, Tim Denyer, describes the task as supplying empathy, but not sympathy.

With the film choosing to also briefly profile several other fellow masochists in the community, it doesn’t fully touch bottom in chronicling Sowerby’s psychological recovery. But perhaps these things can’t be fully explained, only experienced; if nothing else, this doughty saga leaves you glad he experienced it on our behalf.

• Don’t Be Prey is out on 20 March in the UK, 27 March in Ireland and 7 May in Australia.