I think Kisame wins high difficulty, he can absorb a lot of kakuzu's ni ninjutsu / chakra. Also Kakuzu's powerful ninjutsu will be used against him with Kisame's shark bomb jutsu and I think Kisame can absorb. Doton defense because Kakuzu is using Earth chakra to harden his skin.
It's true earth > water but Kakuzu has shown no other earth style feats other than doton defense and Kisame's mass water style techs willl likely to over whelm him in the end. Kisame has to be careful of getting electrocuted though.