Kisame did already have sharklooks before he obtained Samehada. So Samehada is
not the reason for Kisame's looks.
Kabuto summons snakes and doesnt look like a snake (not before he injected Oro's DNA).
Anko summons snakes and doesnt look like a snake.
Same goes for Sasuke.
So having the sumoning contract with an animal is probably not the cause for the looks.
I believe it just has to do with the fact that Kishimoto originally had the idea to make
Akatsuki a group of monsters (he didnt continue that idea). Kisame his design is
probably from that time.
Maybe he uses senjutsu chakra somehow, who knows.
Eating chakra would be an ability of shark senjutsu.
Pretty fitting.