@maxdog,
Holy mackerel: there are world championships in long distance swimming, the "Triple Crown of Open Water Swimming" is awarded to those who did three of the most well-known marathon swims, and nearly every country holds annually Open Water Championships with distances from 1.5 km up to 7.5, 10, and even more km.
Edit: looking up the various results: in the Bavarian State Championship this year, there was even there (Alpine region!) a female youth championship swimming over 7.5 km.