Umibōzu

was used by Amachi to terrorise the Land of the Sea.

Appearance
Umibōzu looked pretty much like a giant water formation with spots that looked like two eyes and a mouth. It was light blue in colour and translucent.

Abilities
Umibōzu itself is nothing more than a gigantic mass of water, that has been gathered, and held together by the summoner's chakra. Being made completely of water, it is virtually indestructible unless it is completely evaporated. The creature can also expand its size with a larger supply of water. Other than being able to morph its form, Umibōzu's main attack method is capturing targets in its water body and drowning them, as well as shooting powerful bullets of condensed water at its opponents.

Kaima Capture Mission
Naruto Uzumaki got swallowed by its indestructible water mass and was completely immobilised, until he gained a burst of chakra from the Nine-Tailed Demon Fox and eventually broke free, shattering it into tiny droplets. During a later encounter, after Umibōzu reformed, Naruto resorted to summoning Gamabunta and it was defeated by Naruto throwing an explosive tag attached to a kunai into Gamabunta's oil, completely evaporating it.

Trivia

 * is a spirit from Japanese folklores.