Lol, middle of the ocean? Kisame wins pretty easily, in both scenarios. Shark attacks or Shark summons give Kisame far greater mobility than what Hidan and Kakuzu have on the water, so evading the elemental jutsu shouldn't be too hard. Hidan's immortality is no issue here since he can be taken out at any time, before or after Kakuzu's death, by Kisame's
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, which will rip and tear into his body. Dismembered Hidan=Beaten Hidan.
Daikodan handles Kakuzu.
The same can be applied in Scenario 2, somewhat. But this time elemental jutsu can be absorbed or dodged, Hidan can be dismembered via Samehada, and instead of having to use Daikodan to kill Kakuzu, he can simply use Suiton and Samehada.