Women reduced the sex difference in open-water ultra-distance swimming: La Traversée Internationale du Lac St-Jean, 1955-2012. There are several well-known long-distance swimming in the open waters.A competition with a long tradition is, La Traversée International du Lac St-Jean ‘in Canada, who has been held since 1955. If you look at the performance of the best swimmers and swimmers since 1955, men like women were faster and the difference was 8.8 ± 5.6% over the years. Looking at the fastest three swimmers and swimmers a year since 1973, men like women were faster and the difference was reduced from 14.4 ± 11.0% to 3.7% ± 1.4%.
In hardly any other occasion in long-distance swimming, women came so close to the men as in this competition. All details can be found under https://pubmed.ncbi.nlm.nih.gov/24476486/