A comprehensive study of 4,085 Canadian beachgoers found that swallowing contaminated water increases acute gastrointestinal illness risk by 20 excess cases per 1,000 people, with E. coli levels serving as the primary predictor of illness. Body immersion alone resulted in 5 additional cases per 1,000 beachgoers compared to no water contact. The research reveals seagull fecal contamination as a key source of pathogenic E. coli in recreational waters. This prospective cohort study fills a critical gap in understanding recreational water illness risks in temperate climates, where previous research has focused heavily on tropical and subtropical regions. The findings suggest current water quality monitoring protocols may need refinement, as traditional enterococci measurements showed weaker associations with illness than E. coli levels. For health-conscious adults who frequent beaches, the data supports avoiding water ingestion and choosing swimming locations with regular water quality testing. However, the overall 2.6% illness rate indicates recreational water contact remains relatively safe for most beachgoers. As this remains a preprint awaiting peer review, these risk estimates may be refined through the editorial process, though the large sample size and rigorous methodology suggest robust findings.