This skill is mentioned in the infamous Shoninki ninjutsu manual of 1681 written by Natori Masazumi. The chapter titled ‘Learning from Waterfowl’ is a collection of skills that center on shinobi infiltration and hiding methods using the element of water.
Boring a hole into my scabbard, I took to training the skill of using it to breathe underwater.
This skill is not as easy to develop as one might expect, particularly if one was to be pursued by samurai henchman armed with man-killing katanas. The water temperature induces shivering that makes breathing calmly through the make-shift snorkel a difficult task indeed.