So you want to make a metaphor in its narrowest sense, something that does not use any explicit word like like, as, resemble, compare, (の)ような, みたいな, 似ている?
Then the solution is simple; use XはYだ/です, and let the listener notice the metaphor. It appears structurally identical to an ordinary sentence, but that's the definition of metaphor (in the narrow sense), after all.
You are my sun.
My father is a demon. (= a cruel person)
(literally) All men are wolves. (= sexually aggressive person, woman chaser)
He has become a tengu. (= big-headed person)
Please keep in mind that each culture has its own set of known metaphors. A metaphor that works in your culture may not work in different cultures. I believe "you are my sunshine" and "you are a demon" work almost in the same way in English and in Japanese. But "you are a wolf" has different implications, and "you are a tengu" obviously does not make sense in English. Conversely, "you are a chicken" makes perfect sense in English, but 君はニワトリだ makes almost no sense in Japanese.
Regarding fish, ～は魚だ ("～ is a fish") is not a well-known metaphor at least in the Japanese culture. People can instantly understand you are trying to make some metaphor, but they cannot get the implication. Still, in creative writings like poems, you can always say 君は魚だ instead of 君は魚みたいだ/君は魚に似ている, and explain it later.