Kisame wins it.
-He outmanoeuvred v1 Bee and Base bee in a CQC bout, it was only when Bee went v2 did he get blitzed, but he still managed to siphon off enough chakra to survive Bee's v2 Lariat. Given this, he should be able to keep up with the 3rd's speed.
- Samaheda has a 360 degree absorption radius, as shown when it absorbed Base bee's raiton projectiles. So, even if the Raikage tries to blindside him, the nukite will lose it's penetrative power and get absorbed, and Kisame should still be able to heal from any damage.
- The raikage could separate Samaheda from Kisame, but that becomes impossible if Kisame even partially fuses with Samaheda, which would make it an extension of his arm basically. There's also water clones and the Water prison technique that could be used against the Raikage.
-Finally, once the water dome comes out, the Raikage is done for. KIsame is faster underwater, and basically sucked the Hachibi dry of chakra. The same thing happens to the Raikage, and he cannot harm Kisame in any way either.
A better match would be the 4th Raikage vs Kisame, because the 4th could blitz Kisame if the starting distance is short enough, unlike the 3rd.