Cliché Storm: Rebellious human daughter flees overprotective parent, makes deal with evil sea witch to become a mermaid, then that backfires as the bad guy nearly takes over the kingdom. In other words, the plot of the first movie, now flipped!
Idiot Plot: The movie could have ended faster if Ariel had just told Melody why she couldn't go to the sea. While it's pointed out several times in the film, it's after the consequences of it, making at best Better Than a Bare Bulb. Keeping her ignorant of her marine heritage wasn't exactly a smart move either since that ignorance is what sets the second half of the plot in motion.
Moral Event Horizon: Morgana either crossed it when she held a baby Melody over Undertow with the intention of letting him devour her or when she gloatingly left a teenage Melody to drown in an underwater cave, mockingly telling her that "her time as a mermaid has just about expired". In both cases, it shows that she's not above killing children.
What an Idiot: Even with the reason of being concerned for Melody's safety, you really have to wonder what made Ariel jump to the conclusion that keeping her daughter confined and entirely ignorant of her ancestral roots was the best idea. Ariel of all people should know better, seeing as her own father attempted the same thing with trying to keep her away from land.
The Woobie: Melody. Her parents (especially her mother) forbade her from going into the sea without an exact reason. And because of her mermaid heritage which allows to do thing like speak to marine life, many of her peers thinks she weird and this leads to her being publicly humiliated at her 12th birthday party, leaving in tears. Gets into an argument with her mother about the locket and why she can't go out to sea and then runs away. She's tricked by Morgana into stealing from her own grandfather and is trapped in a block of ice until the spell keeping her as a mermaid wears off, which nearly drowned her. This girl needs a hug.