Documentaries about driven eccentrics may be a specialty of Werner Herzog, but Louise Osmond and Jerry Rothwell demonstrate they’re up to the task with “Deep Water.” Its subject is Donald Crowhurst, an enthusiastic entrepreneur and amateur sailor who entered an around-the-world solo race in 1968 with disastrous results. It succeeds both as a riveting adventure story and as a portrait of a troubled and mysterious man.

As the canny mixture of archival footage and sound recordings, contemporary interviews and explanatory narration and newly-shot footage explains, the race was a challenge typical of its time, issued by The Sunday Times with a handsome prize attached. Following a successful solo circumnavigation of the globe by Francis Chichester, a much-feted gentleman who’d stopped for refitting along the way, the paper offered cash awards to those who could do it without any landfall—the person who did it first and the one who did it fastest would each receive recognition.

Most of the men who chose to enter were experienced sailors, including Robert Knox Johnston, who appears in both old footage and in contemporary comments, and Bernard Moitessier, a Frenchman with a mystical love of the sea about whom his wife Francoise reminisces with a typically Gallic mixture of ebullience and philosophy. Crowhurst was the old man out—a weekender, as some later called him, a small-time maker of sailing equipment who persuaded a land-lubbing sponsor to put up the money for an experimental yacht and found a bulldog of a publisher who adopted him as the linchpin of his coverage. He was also a fellow with a loving wife and four small children, as well as looming financial problems.

As the picture shows, Crowhurst wasn’t cut out for the race, but once he’d committed himself to it, he had no choice but to set out despite a host of setbacks and stay in even though, as film he took of himself on the journey and the reconstruction of his voyage demonstrate, he quickly ran into trouble. It would be unfair to reveal how the race proceeded—the tale is hardly an unknown, with plenty of books and articles written about it—but suffice it to say that there are plenty of surprises along the way, including a couple of dillies near the close that forced Crowhurst to make some fateful decisions.

One might expect a story like this to reach for some uplifting moral, but “Deep Water” prefers (as “Grizzly Man” did) to emphasize the awesome natural forces that some human beings decide, for whatever reason, to pit themselves against, and the fact that nature is not always benign in its response. More importantly, it paints incisive portraits of the important actors along the way—Crowhurst of course, along with his wife Clare (whose modern commentary, delivered with stiff-upper-lip restraint, is quietly moving) and his doting children (one of whom, Simon, offers some wrenching recollections), but also friends, journalists and fellow competitors. This isn’t primarily an adventure story, though it’s very suspenseful; it’s first and foremost a profoundly sad tale of human frailty.

Technically polished, with expert editing of the diverse components by Justine Wright and Ben Lester, a finely-tuned score by Molly Nyman and Harry Escott, and appropriately low-key narration by Tilda Swinton, this is yet another documentary that proves that real life can often be more compelling than Hollywood fiction.