The Little Mermaid II: Return to the Sea

Ariel and Prince Eric are happily married with a daughter, Melody, who is a human-mermaid hybrid. To protect her from the Sea Witch, Morgana, they have not told her about her mermaid heritage. Melody is curious and ventures into the sea, where she meets new friends, including Tip and Dash, a penguin and walrus duo. Morgana threatens to have Melody fed to her tiger shark sidekick, Undertow unless Triton gives his trident. Ariel and Eric work together to foil Morgana's plan, and Triton transforms Undertow into a piranha. Melody eventually discovers her true identity and uses the trident to destroy the wall separating her home from the sea, reuniting the humans and the merpeople.