Marina sings a dark reprise of her song on the night of her death.
The ending to the anime is possibly worse than the short story. Right after Marina jumps into the sea, the Prince remembers her... but it's too late. Fritz is left to tearfully chase the sea foam that was Marina.
Made even worse when, as an adult, you realize that outside of a Disney universe, what Triton did qualifies as straight-up emotional abuse. Destroying your child's property, even in anger, has the potential to scar them for life. It doesn't help that it easily allowed Flotsam and Jetsam to take advantage of her either.
Sebastian's grief over the whole thing, as he indirectly caused the incident when he accidentally blurted out Ariel's secret.
Sebastian: Ariel, I—
Ariel:Just go away.
Even Triton seems to have realized what he has done was too far witnessing her sobbing. His face after seeing what he's done and broken her heart is awful.
When Ursula wages a deal with Triton to hand the kingdom over to her to get Ariel back. Seeing how his own actions caused this chain of events, he sadly hangs his head down and willingly signs the contract.
Ariel excitedly running down stairs, only to see Eric being romantic with "Vanessa".
In a strange sort of way, Ursula mourning the loss of her minions and the Mama Bear-fueled rampage she commits afterward.
The entirety of the ending. Yes, all of it. Though, in this case, it's more of a case of happy tears. It's the one moment that you realise that Disney, despite a few films that we do not mention, can do tearjerking scenes!
Triton finally realises that Ariel does love Eric and changes her into a human, no deals made and no strings attached, so that she can be free and happy, no matter how much it hurts him while bells toll victoriously in the background. The look on Ariel's face is one of sheer and utter joy and Triton smiles sadly, yet happily for his daughter. And Sebastian - he's so happy!
Triton: Then, I guess there's just problem left.
Sebastian: And what's dat, Your Majesty?
Triton: *sadly* How much I'm going to miss her. *Changes Ariel into a human*.
Max licking Ariel's face after she and Ariel are married; this time, there's no fear from her - and he's utterly and completely thrilled to be correct - he knew Ariel was the one who saved Eric and he's done it, they're together!
Carlotta crying into Grimsby's tie.
Every single merperson and many aquatic creatures that knows Ariel, along with all her sisters, waving goodbye; one of them actually looks to Triton and shares a smile.
Flounder's sad face and Scuttle's wink as the two return to the sea after Ariel kisses the former and ruffles the hair of the latter.
Sebastian, holding the bride and groom figurine on the wedding cake, with tears in his eyes.
Triton rises from the water to embrace his daughter - with a little hesitation that is quickly broken - and then, Ariel says the one line that - if you heart already wasn't broken - breaks your heart:
"I love you, Daddy."
The look on Triton's face after he and Ariel separate and, a Triumphant Reprise of "Part of Your World" blasts at us (Now we can walk, now we can run, now we can stay all day in the suuuuunn, just you and me, and I can be, part of your woooorrrrrlllllllddddddd). Then, Eric bows to Triton and the Sea King bows back with Ariel smiling happily. Yes, he bows to a human - a member of the same species that killed off his wife, showing just how much Triton has changed. Then, he cups his daughter's face gently and lowers himself back to the sea... while his daughter blows a kiss.
Finally, as the boat leaves and the happy couple waves, Triton fills the air with a rainbow and Ariel and Eric end the film... with true loves kiss.
Return to the Sea
Ariel finding out Melody ran away.
This scene from the beginning.
Ariel: This is the only way. Melody can't know anything about merpeople or Atlantica...or even you, Daddy.
The image of Melody being changed back into a human while in the cave and nearly drowning as a result.
Melody being humiliated at her birthday party, calling herself the princess of disaster, and when she hears the guests talking meanly about her. Poor girl.
Triton watching the death of his beloved queen in a pirate attack, followed by him throwing away the music box far into the sea (which Sebastian secretly witnessed).
Sebastian getting arrested.
Triton lying down all sad with a statue of him and Athena in the background.
Ariel does the exact same thing in her room, and decides to slip out as a piano version of the music box tune plays.
Ariel finding the music box and remembering her mother.
Sebastian: When your mother died, the whole kingdom was heartbroken. The heart that never healed... was your father's.
Ariel: It's so sad... He's forgotten what it's like to be happy.
Triton's heartbroken singing when he thinks Ariel is dead and cradles her in his arms.
The line that makes Triton realise that, maybe, things are a bit wrong:
Ariel: I may not remember my mother... but I know she wouldn't have wanted this!
The TV Series
The character of Gabriella, a deaf mermaid. (She was based on a real deaf girl, a fan of the show who died during the first season's run.) In her first appearance, "Wish Upon a Starfish", she's heartbroken to realize that the great wishing starfish can't actually grant wishes, which means that she'll never be granted a voice. As it turns out, she doesn't feel she can fully express herself without one. Luckily, Ariel reassures her that her sign language is as beautiful and expressive as speech would be.
The episode where Ariel feels she can't go home because a bracelet got stuck on her arm and she doesn't want her father angry with her.