Halle is brilliant, and her presence is extraordinary, but not phenomenal. That said, she is a powerhouse, and with the proper script, she is going to shine even more brightly.
It's not perfection, and there is little new to the story unlike Aladdin for instance, but the performances, the development, the photography, and especially the music are particularly captivating and make for a fun, lightweight, and still true to the original animation film.
The script could have certainly used a bit more tweaking here and there, and I would have loved to see more of Ursula and her counterpart Vanessa, seeing as they were extremely threatening in the original one. Still, McCarthy does not disappoint.
Another character that should have seen more development would have been Triton, though I do understand why they wouldn't have elaborated more on him. After all, the film is titled "The Little Mermaid", not "The Tale of Triton".