Sōsuimyaku

Sōsuimyaku (蒼水脈, Literally means "Azure Water Pulse") is the ability of Water Clones or Hōzuki to alter/modify their form at will, due to their hiden technique or in the case of clones, their composition.

Overview
Being made of water, the user has quite a deal of versatility when it comes to manipulating their form. As a result, they can alter and create specific parts of their bodies or new forms altogether for weaponry, defensive measures and more. In a similar vein to Kekkei Genkai like Shikotsumyaku, the user could manipulate their body to form structures similar to those of the Kaguya, though obviously not made directly from bone, but water.

These structures as seem to become real, once formed. Given the ability of water clones to assume a form similar to a Shadow Clone, this isn't that far off from that. As seen with the Steaming Danger Tyranny, the axe formed was able to be used offensively and realistically. This gives this shape-shifting great fame and one of the reasons the Hydrification technique is very reputed.

This shape-shifting does not end with shape alone. For skilled Hōzuki or water clone users, the user can assume a form in another state of water, such as mist, steam or even ice, allowing them to manipulate themselves even in that form. While this hasn't been seen much, it is assumed that can be used for a user to enter the opponent because the opponent breathes part of them in, allowing them to do what they will with the body. This is what Gengetsu used simply to create his smiling chibi face in deadly hot steam for an intimidation factor for his technique.

Trivia

 * Thanks to Ash for the translation of this technique.