Suiba (水馬)
Suiba was an annual event in the Edo period held by Edo bakufu (a Japanese feudal government headed by a shogun) in which people rode horses across water.
Suiba was also called "Umakawawatashi" or "Umawatashi."
Summary
Suiba was held around June or July at the Sumida-gawa river with the participation of several dozens of samurai; namely, the shoinban (castle guard), the okoshoban (official guard in the Tokugawa Shogunate), the oban (a group of guards in the Edo shogunate), samurai working at one of four stables (in Tsurumi, Magaki, Suwabe or Urabe), and samurai working for one of the three families related to the shogun (the Tayasu, the Hitotsubashi or the Shimizu).
The horse riders wore a white, dark blue or light yellow mizu-hanten (a kind of short coat for a in-water use), made of weathered hemp cloth with Kamon (family crest) dyed on it, sewn with taihaku ito (a thick white silk thread). Their horses were wrapped tightly with a hemp cloth around the abdominal area. First, the horse was led to the water by its rider holding the right side of its neck. When the water got too deep for the horse to touch the bottom of the river, the rider moved back until he was sitting on the horse's buttocks and drove the horse forward using reins attached to rings on the hemp cloth.
Experienced riders wore a short mizu-bakama (a pair of trousers worn over a kimono in water) and a mizu-hanten, and they also put a mizu-kura (a saddle used in water) and mizu-yoroi (armor used in water) on their horse. When the water got too deep for the horse to touch the bottom of the river, the rider dismounted from the horse at its neck, helped it with going forward holding the bit, and then the rider moved behind the horse to drive it on. The rider mounted the horse again when the water got shallow enough for it to stand as it reached the opposite bank of the river.
The most experienced riders could do this while wearing a suit of armor.
A shogun's attendance at the suiba was called "suiba joran." The first suiba joran was on August 30, 1735 attended by the shogun Yoshimune TOKUGAWA.
It is said red lacquered vessels chartered by the Edo bakufu guarded the area.
Suiba was adopted as a drill of the cavalry department of the Imperial Japanese Army and became an important in-water exercise.