Duke Kahanamoku, popularizer of modern surfing, won the 100-meter freestyle at the Swimming Championships in which European country in 1912? Hint: He participated and won in a national swimming championship shortly after the Olympic Games in Stockholm.

It appears to have been Hamburg, Germany, in a straightaway, open fresh water event on 20/7/1912. and 21/7/1912. He set a world record of 1:01.6 in the first swim and tied it in the second. There seems to be very little information easily accessible online about the event.

Duke went on to set numerous further world records across various distances and conditions and was able to lower his 100m mark further.

https://en.wikipedia.org/wiki/World_record_progression_100_metres_freestyle
https://www.outriggercanoeclubsports.com/olympians/duke-kahanamoku/