This doc is not a colorful geezer story, though its first half-hour is shaped like one. The fiftysomething Slovenian marathon swimmer Martin Strel has a boxer’s broken nose, a massive wine belly, and a compulsory grin that breaks out whenever a camera heaves into view, preferably with a corporate sponsor bringing up the rear. Strel looks like the top-heavy dad in The Incredibles, but though he’s trying to swim the polluted Amazon with an entourage almost as insane as he is, it turns out he’s more Grizzly Man than Superman. Filmmakers John Maringouin and Molly Lynch have their own eco-agenda and a rather too poetic way with Peruvian sunsets, but they’re not buying Strel’s claim that he’s pulling this stunt to save the world’s depleted rain forests. Strel loves a good horse-burger almost as much as he loves a media circus, and Big River Man is plausibly, if melodramatically, told as a parable of a damaged man who’s trying to outswim his demons (gambling, drinking, a horrible childhood) or trying to commit suicide slowly—or both. The Big Swim, with its alligators and parasites and all, is fun. But the real drama lies in the sweetly twisted symbiosis between this likable, infuriating wreck of a man and his devoted son and publicist, Borut, a pliant computer programmer who could be Matt Damon’s double with a little tooth repair, and who has his own river to cross.