Granted, sea life on Earth is incredibly diverse as well so it wouldn't be too surprising if aquatic life on another planet looked like SOMETHING we had on Earth.
Right. There is nothing special about the development of fins and extended tails.
We might find a new paradigm that hasn't manifested or survived on earth (10 legs spin kicking gracefully to propel), but that is way more complicated than muscles and hard skin, so it's unlikely.