This study examined participation and performance trends in the 26.4-km open-water ultra-swim "Marathon Swim in Lake Zurich," Switzerland. A total of 461 athletes (157 women and 304 men) finished the race between 1987 and 2011. The mean age of the finishers during the studied period was 32.0 ± 6.5 years for men and 30.9 ± 7.2 years for women. The mean age of finishers and the age of winners increased significantly across years for both sexes (p < 0.01). The winner times were significantly lower for men (403 ± 43 minutes) compared with women (452 ± 63 minutes) (p < 0.01). In contrast, the mean swimming time of the finishers did not differ between men (530 ± 39 minutes) and women (567 ± 71 minutes) (p > 0.05). The swimming time performance remained stable (p > 0.05) for both sexes across years. A higher age was associated with an increased risk for not finishing the race (odds ratio = 0.93, p = 0.045). Swim time was negatively associated with water temperature in the top 3 swimmers (ß = -9.87, p = 0.025). These results show that open-water ultra-swimming performance of elite swimmers over 26.4 km in a freshwater lake is affected by age, sex, and water temperature. The sex difference in open-water ultra-swimming performance (approximately 11.5%) remained unchanged these last 25 years. It seems unlikely that elite female swimmers will achieve the same performance of elite male swimmers competing in open-water ultra-swimming in water of approximately 20 °C. Anthropometric and physiological characteristics such as skeletal muscle mass and thermoregulation need additional investigations in female and male open-water ultra-swimmers.