Sui-ren

Sui-ren (水練, suiren) 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

Special boats and floats were used to cross over water, known as hasami bune.

There are also techniques used to cross medium rivers that could not be jumped

Water as a weapon

Ninja used water as a weapon, destroying bridges, dikes, or contaminating water sources, or withholding water from captives. They would weaken the bridge or dikes support beams so when an enemy walked over it would collapse.

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 under 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.

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.