This film really went away from the source and cast people that don’t look anything like the characters at all. Even Prince Eric was too old (28) and too tall (6 ft 2). Also I think Melissa McCarthy has played too many comedic roles to be seen as a villain. There's just certain roles that someone is able to take if they've made so many, many lowbrow comedies. The only casting choice I liked in the whole film was Flounders.
McKenna Grace as Ariel
Kathy Bates as Ursula
J.K. Simmons as King Triton
also
Doug E. Doug as Sebastian
Jack Black as Scuttle
and Jacob Tremblay as Flounder