Objectives To provide a rationale for minimum water temperature rules for elite and subelite marathon swim racing and highlight factors that make individuals vulnerable to excessive cooling during open water swimming.

Methods 12 lean competitive swimmers swam for up to 2 hours, three times in different water temperatures between 14°C and 20°C, wearing standard swimming costumes and hats. Rectal temperature (T_re), oxygen consumption, perception of cold and performance were measured.

Results In 16°C, half the swimmers did not complete a 2-hour swim; four became (or were predicted to become) hypothermic within 2 hours. In 18°C, three-quarters completed the swim; three became (or were predicted to become) hypothermic. In 20°C, one swimmer was predicted to become hypothermic in under 2 hours. The mean linear rate of fall of T_re was greater in 16°C (−1.57°C/hour) than 18°C (−1.07°C/hour) (p=0.03). There was no change in swimming performance during the swims or between conditions. Most of the cooling rate could be explained by metabolic heat production and morphology for both 16°C (R²=0.94, p<0.01) and 18°C (R²=0.82, p<0.01) conditions. No relationship was observed between T_re and perception of thermal sensation (r=0.25, p=0.13), and there was a weak correlation between T_re and thermal comfort (r=0.32, p=0.04).

Conclusion We recommend that 16°C and 18°C water are too cold for elite marathon swim racing. FédérationInternationale de Natation rules were changed in 2017 to make wetsuits compulsory below 18°C and optional below 20°C.