Chronicle Staff Writer San Francisco Chronicle, Copyright 2010 It's easy to imagine filmmaker John Maringouin whooping with joy when he came across the story of long-distance swimmer Martin Strel. Strel is a grinning, publicity-loving Slovenian who sets out to swim the length of the Amazon -- more than 3,200 miles -- though he's over 50, has an enduring love for booze and possesses the beefy body of a wrestler past his prime. You'd dismiss Strel as nuts, except he's already swum Mississippi, the Danube and the hideously polluted Yangtze -- all when he was in his 40s. The proclaimed reason for his Amazon adventure is to raise awareness about the rain forest, but the film makes it clear that more personal issues (such as childhood abuse) are at stake. The film is narrated by Strel's son, Borut, who functions as his father's manager and personal assistant, even going as far as to impersonate him in telephone interviews. We learn that Strel senior, a former professional gambler and teacher of flamenco guitar, is beloved in his homeland, where the police allow him to drive drunk. We also see father and son enjoying a tasty local treat, a horseburger. That's the preamble. Most of 'Big River Man' follows Strel's in-the-water experiences in South America, where he faces crocodiles, parasites, severe sunburn, whirlpools, killer floating logs and some kind of fish that enters the body through the urethra. His navigator, an American named Matt Mohlke, is in way over his head, and confesses that he's just learning to use a GPS device. The swim begins to take its toll, and not just on Strel. The combination of exhaustion and alcohol-related dehydration leave him barely able to move when he's out of the river. Toward the end of the epic swim, his doctor says his heart is overtaxed, but Strel refuses to stop. He takes off by himself at odd times, and begins to hallucinate. The fever seems to infect Mohlke, who starts ranting that Strel is Christ-like and 'the last superhero.' It's tempting to keep piling on the details, but that would spoil the picture. Suffice it to say that the film ends on an unexpectedly ecstatic note. Strel is one strange duck, and you can only wonder that Werner Herzog, with his fondness for captivating weirdos, didn't get to him first. 