As someone who adores The Little Mermaid, this retelling fell flat for me. The concept had a lot of allure with Ursula having won and exploring what happens after, but the pacing really dragged, and lacked the magic and wonder I was hoping for. Even though the stakes should have been high, they felt surprising low for most of the book. 
I think readers who aren't as close to the original Disney movie may have a better time with this one. For me, the tone just didn't land the way I'd hoped.