Hiroyuki Arakawa is a scuba diver in Japan. Yoriko is a fish. They have been friends for 30 years, seeing each other almost every day.
From Twisted Sifter:
One day, Arakawa found her looking exhausted and carrying an injury. So he did what any friend would do: he took care of Yoriko, feeding her crabs and nursing her back to health. Their decades-long friendship is proof there's no greater bond than the one between man and fish.