Tanjiro’s Water Breathing says MIZU NO KOKYU in Japanese.
MIZU = water
NO = of
KOKYU = breathing
Water Breathing is one of the Total Concentration Breathing and has the oldest history among the five breathing styles of the Demon Slayer Corps.
It’s only Water Breathing that has a technique to kill demons without giving them pain and give peaceful death. I think this feature really matches Tanjiro’s merciful personality.
It requires calm and quiet mind like surface to enhance the skills of Water Breathing.