Sui-ren was the training that ninja underwent for techniques involving water. Some ninja were tasked to deal oppenents either with the water, disguise themselves with it, or must 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 long in the water, but kept the ninja afloat for long periods of time.

Water as a weapon

Ninja used water was a weapon, destroying bridges, dikes, or otherwise contaminating water sources, or withholding water from captives.

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 oppenents from the water. Ninjas also developed a 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.

Survival

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