The Little Mermaid II: Return to the Sea (2000)

Morgana, sister of Ursula, almost steals the baby Melody from her parents, Ariel and Eric. To protect Melody, she's told never to enter the ocean, but by her 12th birthday, she's been swimming there for years. She discovers the magic locket that was to be her christening gift, which makes her visible to Morgana. Morgana tempts her by offering her fondest wish, to turn her into a mermaid; however, to remain a mermaid, she'll have to steal the magic trident from King Triton (who, unknown to her, is her grandfather). Meanwhile, all the sea creatures are searching for her. Along the way, she joins up with comic relief sidekicks Tip (a penguin) and Dash (a walrus). When she returns with the trident, Morgana's evil nature is revealed, and there is a massive battle of good and evil.