It is a festival which negotiates about hot water only in the shape of fundoshi.
About Fundoshi (Wikipedia)
Fundoshi (褌) is the traditional Japanese undergarment for adult males, made from a length of cotton. Before World War II, the fundoshi was the main form of underwear for Japanese adult males. However it fell out of use quickly after the war with the introduction of new underwear to the Japanese market, such as briefs and trunks.
Nowadays, the fundoshi is mainly used not as underwear but as festival (matsuri) clothing at Hadaka Matsuri or, sometimes, as swimwear.