Suiren is a term best translated as “aquatic skills” or “aquatic training.” These were skills to help a shinobi, samurai, or foot soldier when swimming. Shinobi were trained to dive and swim and they could spend long periods of time in water without breathing.
Natori-Ryu has a raft made out of a square from four bamboo sticks, with each corner tied together, and a bottle gourd on each corner. Moreover, a simple tub could be used as a floatation device.
According to the Shoninki shinobi manual, when hiding underwater a shinobi should allow his face to surface when he is hidden by the shadows of the trees, while drawing reeds around him and his face to hide his face and body. This will allow him to breathe and stay hidden. For the shinobi who are adept at diving, they should drill a hole in a bamboo stick or in their scabbard to breathe through it from time to time while diving, taking breathes as they come close to the surface.