This is a story that I think I lean towards more than others at times. I feel there is so many things you can do with it. I think this would be a platform where You can be extra creative with the fact most of the story is set un the sea.
Some quick ideas I had to rework the story and sticking with what I mentioned earlier, tell the story from the villains perspective. King Triton and Ariel could be devious characters and Ursula be a victim. Ursula could have once been queen of the sea until Triton overthrew her in his evil plans to become ruler.