Had a look at the IMDB -
https://www.imdb.com/title/tt5971474/
Hermione from the Harry Potter Play is playing someone called Queen Selina - Ariel's mother, I'm guessing?
Melissa McCarthy is Ursula - cool, usually like her.
Jacob Tremblay as Founder. Sure, why not, he's on everything these days.
Lin-Manuel Miranda is rumoured to be playing the Chef. Bit of a waste, he's too good for such a small role really.
Don't know the rest.