Fish and whales are what they are because of the series of random mutations that occurred starting with the initial life on Earth. A similar string of mutations occurring in another part of the universe seems very highly unlikely to me, so fish and whales being on Enceladus would blow my mind. From what I understand, it's most likely that life in other places won't resemble life on Earth except maybe the most primative organisms.
But you can expect similar functions to evolve, like mouth and anus, likely eyes, and possibly other senses, which in a primitive state, may be very similar to in simple earth organisms.
Whether or not things like bone structure evolve, physics is the same across the universe, barring differences like planet gravity, etc, so the need for locomotion could result in very similar structures to, on a larger scale, fins, legs, wings.
Think of how similar sharks, dolphins, and extinct marine reptiles are structurally, because of their shared environments, despite remarkably different ancestors.
So I think if the day arrives when we do discover life (on an earth like planet) we'll possibly see more similarities than differences, again, assuming similar origin conditions.
But yeah, it will never ever be the discovery of fish, or humans, or anything in the exact biological structure as on earth.