Excuse my ignorance but I’d never heard of Underwater Orienteering before I came across the stamps featured here! Organised in bodies of open water over courses of various shapes and distances it is a combination of underwater diving and orienteering. The sport was developed in the Soviet Union during the late 1950s and competitions are currently played mainly in Europe. Known as Orientation Sub in French and La Orientación Subacuática in Spanish. Above is a stamp from 1968 produced by the Soviet Union in recognition of the Scuba Diving European Underwater Orienteering Championship held in Alushta, Crimea, that year.
Held every two years and organised by the Confédération Mondiale des Activités Subaquatiques (CMAS), the Underwater Orienteering World Championship is the highest international competition. The stamps here, issued by East Germany (DDR) in 1985, commemorate the 2nd World Championship in Underwater Orienteering held at Neuglobsow, DDR, in August 1985.
Below is an article from January 2018 with Orienteering as the theme.