Yes he can breathe underwater and absorb chakra without samehada. Samehada is basically an extension of his power. Basically a way to absorb chakra from a distance without using chakra. Samehada also acts as a container as kisame can use chakra to heal himself, so having stored up chakra in samehada is useful . If he runs out of chakra. Boom free replenish. If he gets hurt so bad his own chakra alone isn't enough to heal him then boom, free revive from samehada. And when he fuses with samehada he basically enters sage mode in the form of sage mode transformation (jugos ability) and grows into a WereShark. Now of course he doesn't have senjutsu chakra but he does have his own chakra absorption abilities and samehadas stacked on top of each other. Kisame has shown to take samehadas chakra from it against it's will while it was clung to bee when he was leaving the waterfall place. Meaning he can absorb chakra at a greater rate than samehada. Samehada can absorb up to 6 tails worth of tailed beast chakra at a time or a whole entire V2 shroud including the chakra skeleton. With just one contact. So while formed that's 12 tails worth of chakra cloak or two V2 cloaks or even more. He drained killer bee and the 8 tails completely dry (ableit already absorbing most of bees chakra) so he drained the 8 tails while he was basically still at full chakra even if not, he had the same amount as he does fully if you add what bee had left onto his when kisame fuses. And then kisame drains him dry with like 6 hits. That's it. Like holy FU@#_#. If he isn't fused you must be able to use a move with more than 6 tails worth of chakra or u won't be able to do any damage cause it'll be absorbed. Now let's say u use an attack with 8 tails worth of chakra. So 2 tails worth of chakra gets through and busts up kisame.... Lol well now he has 6 tails worth of chakra to heal himself. And he had all of his organs in his abdomen, liver, kidney, muscle tissue, etc. Blown away and REGENERATED it all. And then if he's fused ... Lol ... Only people like the first hokage, bm Naruto, EMS Sasuke and Madara, maybe pain, maybe sage mode jiraiya and Kabuto due to senjutsu, guy, and anyone stronger than them can even hurt him and if they do well than Wala he has 12 tails of tailed beast chakra cloak chakra or more worth of chakra to heal himself with. Like... I almost wonder why he didn't have a water style chakra armor like the raikages lightning , except it could instantaneously absorb chakra and make kisame stronger/weaker down to base whenever he heals himself. Missed opportunity on that one. He'd basically have a healing chakra chakra armor. That would be DOPE
