Part of Your World by Liz Braswell, 475 pages

What if Ariel had never defeated Ursula? Five years after the (twisted) events of the film, Ariel is now the queen of Atlantica, and Ursula is running Eric's kingdom on land. But when the sea witch threatens Atlantica once more, Ariel finds herself returning to a world (and a prince) she thought she'd left behind forever.