They didn't show if Kisame was using Fusion mode.

Kisame, he admitted that he had hard time to knockout Roshi, therefore mostly he had to use fusion mode to take down Roshi.

Roshi and Son have special bond as good as Bee, but not close. Forty years they were together, no way without doubt Kisame wouldn't use Fusion.

At the end of the fight, there was showering or rain pouring down. That would assume Kisame was using his water prison.