Why would KB fuse with samehada? im pretty sure it was a technique unique to kisame and samehada considering kisame was already part fish... KB might have a big bad ass octopus inside him but he cant breathe under water as shown in the kisame - KB fight :/ would have been cool to see tho... even though it is completely pointless
Plus KB is using samehada effectively by using samehada to store some of his swords and using it to absorb chakra while he isnt in tailed beast mode.