David, in New Zealand there is no difference in pronunciation. The plural of fish is generally fish, not fishes, as Caroline said. Your other word, fish's is the possessive e.g. the fish's tail, and the way we pronounce that word is fish + is.

Hi David 大胃王, There is no difference. I'm a native English speaker and would never say /'fɪʃs/. I would pronounce both "fish's" and "fishes" as /'fɪʃəz/. The normal plural of "fish" is just "fish" though, so there is almost never confusion between "fish's" and "fishes" because we rarely use either of them.