Battle of the Death inside the Water Prison!! (水牢の死闘!!, Suirō no Shitō!!) is chapter 472 of the Naruto manga.
Kisame being more agile in water once fused with Samehada; creates a large water dome to fight Killer B within. B cannot survive under water for long and Samehada continues to absorb his chakra, so he tries to escape, but is followed and attacked by Kisame. Once B is exhausted, Kisame separates from Samehada and tries to cut off B's legs to make him easier to carry. Samehada, having grown fond of B's chakra, stops him and protects B, even giving him back some of his chakra. Kisame kicks it away and tries once again to cut off B's legs.