Sui-ren

Sui-ren was the training that ninja underwent for techniques involving water. Some ninja were tasked to either deal with opponents in the water, disguise themselves in it, or to cross it.

Ninja water techniques fall into 5 major categories.

Boats

Ninja sometimes used rafts or portable collapsing boats to follow convoys in the water, or to cross rivers, or in the ocean. These boats were not designed to last forever.

Water as a weapon

Ninja used water as a weapon, destroying bridges, dikes, or otherwise contaminating water sources, or withholding water from captives. They would weaken the bridge or dikes support beams and when an enemy walked over it it would collapse. Special water explosives were even used to splatter water and create a distraction.

Tools

A ninja could use a breathing tube that looks like a reed to stay underwater for a long period of time. These tubes sometimes were used to blow darts at unsuspecting opponents from the water. Ninja also developed special tabi boots that allowed them to walk on calm water. Some ninja clans opted to use blocks of wood tied to their suits to float like a life preserver in the water.

Stealth

In old Japan it was not uncommon for ninja to use water urns, wells, and rivers to hide and watch people. Using the method of breathing through a reed they went into the well filled with water to better conceal themselves. One attack used was hiding in a mostly empty well that was owned by an enemy and when a servent pulled up water the ninja would jump out and take the servant as a hostage.

Survival

Ninja had to know how to find, purify, and transport water over distances. As a survival skill some ninja could find water in unlikely places.