There are many fish in the sea. And when divers come to admire them, it may not just be the divers that gesture. There are some, like octopi, squids and cuttlefish that signal through colour changes. Or they blow themselves up in response to perceived threats, like terrorists pufferfish.
But I once heard a very nice story
About a gesturing fish named John Dory
Who when faced with his fate
Goes head on, stays straight
Then flashes his evil eye to thee
Thanks to EA for the links above and below and the following anecdote: When you get near a John Dory he will first face you directly, perhaps hoping you cannot see him because he is so thin. If you come closer still he turns and displays the extra eye on its flank, pretending he is a big fish. It’s a clear signal to bystanders: “Bugger off”.
But beware: “Compared with the variety of human responses, however, that of a fish is stereotyped, not subject to much modification by “thought” or learning, and investigators must guard against anthropomorphic interpretations of fish behaviour.” (but then again, these image scoring fish appear none too backward).