The Little Mermaid/WMG

Ursula became an octopus because she created Horcruxes
Supporting the theory that Vanessa (with a mermaid tail) was her original form, she dabbled into dark magic to increase her powers, jealous as she was of her brother Triton and all the recognition that he got. To assume immortality she created horcruxes, which eventually distorted her body until she sprouted a few more tails and became the fat octopus we see today. It makes sense in every way; she hid in the Leviathan corpse because nearly all of her horcruxes had already been destroyed (hence her line "And now, look at me - wasted away to practically nothing"; she was referring to her lack of Horcruxes). It also explains why she was so horrified when her eels died (guess why their eyes were so vibrant? They were Ursula's personal Naginis!) and that she died only after the eels, her final Horcruxes, were destroyed. (Triton's trident was probably the only thing that could destroy them anyway).

Marina from the Little Mermaid III trailer is Ursula
She shares a similar attitude. She has a similar hairstyle and she is shown sitting on a chair that resembles Ursula's tentacles. Her breasts are covered by what looks like a purple version of Ursula's black dress in the first film. She is shown sending eels out in a attempt to murder Ariel (which obviously failed, as the film is a prequel). Triton will find out about this murder attempt, use the trident to turn her into the octopus form she is in the movie and TV series, and then banish her forever. This sets up her actions in the first film and reveals why she said "When I lived at the palace". Since she and Sebastion appeared to argue a lot, this would explain her particular hatred for him she showed in the TV series. Watch the trailer yourself and decide.
 * Hm. That trailer has pirates in it. Pirates, eh?

Ursula is Triton's sister, and thus Ariel's aunt
In a deleted scene in the first film, she is explicitly stated as Triton's sister. If, as theorized above, she is Marina from the prequel third movie, that would explain why Triton apparently trusted her to babysit all his daughters.
 * In the Broadway Musical, her being Triton's sister is canon.
 * It was also in the Disney book.

Marina from the Little Mermaid III trailer is Ursula and Morgana's mother.
Basically everything from the theory above, only with the addition of her being the mother of Ursula and Morgana. When Triton cursed her, he also cursed her younger daughters as well.
 * Impossible. She is obviously younger than Ursula and Morgana. Even if she were not, she couldn't be old enough to be their mother.

Marina from the Little Mermaid III is Queen Athena's sister.
Hence how she acquired her job despite her ditzy qualities. It's worthwhile to note that some of Triton's daughters resemble Marina, in backing up this theory. Andrina in particular has her hair, eyes, tail... and dry humor. (When Marina takes off her wig, tiny strands of golden blonde hair are shown.) Oh yeah, and the best evidence is that Marina and Athena have the same eyes.

Sophie is the second chance of Marina Del Rey. Vlad is Benjamin the Manatee.
Watch both Anastasia and Ariel's Beginning with this in mind, and tell me you don't see the resemblances. Marina even had hair of Sophie's color before it fell out. As a bit of trivia, in real life, Sophie's character's inspiration was governess to the Romanov daughters before the revolution, just as Marina was governess to King Triton's daughters. My theory with the animated worlds is that someone believed Marina deserved a second chance at happiness and living a spirited life as opposed to rotting in jail or whatever her final fate was (even if she did become Ursula, which is something people have split opinions over). The mermaid movies end right around the time Vlad and Sophie were born, too. Perfect time for reincarnation.

The music ban in the Little Mermaid III trailer is a metaphor for prohibition
Sebastian's club looks an awful lot like a speakeasy of 1920's America, doesn't it? Especially given the rather ruthless policing of music outside of the clubs...

Sebastian's club in the third movie is the local equivalent to a gay bar.

 * I'm glad someone else thought so too!

Captain Hook was responsible for the death of Ariel's mom.
Stay with me here. In the prequel, that cove looks a lot like the mermaid cove in Neverland. And the ship flies a pirate flag. Who's the local pirate of Neverland? Captain Hook. Ergo, Captain Hook indirectly caused the events of The Little Mermaid.

The seashell bras were for structural purposes, not modesty.
Since getting slapped in the face with a bag of fat every time the mermaids with large breasts (or really fat mermen) turned forward and down would be rather uncomfortable. That's why the only other clothing was a crown, and Ariel had to be told by the characters who were more familiar with humans to get some clothes on. Triton and the other mermen didn't have them because they didn't have anything to keep in place, not because of some double-standard of public decency like with humans, and Ursula didn't need one because of her altered physiology as an octopode.
 * Isn't that the reason bras were invented in the first place, as an alternative to the constricting corset?
 * Yes, though not specifically for the corset. How does that make Did Not Do the Research applicable to the guess?
 * Besides, in the prequel baby girl mermaids wear straps of cloth around their chests, ala the wrap Lilo wears when she goes hula dancing. It's probably the same in our culture- both for support and modesty, as bare male chests are far less controversial than bare female chests.

Ursula is the underwater version of a Wendigo
Cannibals are the most likely to become wendigos. Ursula is the only octopode we see (at least in the first movie), and the only humanoid sea-dweller seen to eat other sea creatures. In addition, wendigos are said to be creatures of great spiritual power, which would explain Ursula's magical abilities. Given her tendency for scheming, she might have broken a fish-eating taboo intentionally in order to gain power, and become banished as a result of being found out.

Triton hates (originally hated?) humans because he doesn't/didn't know that humans aren't cannibals.
Most fish eat other fish; it stands to reason that merfolk, at least on some level, practice cannibalism. There are lots of different types of merfolk, and Triton only seems to have proper dominion over ordinary fish-folk (unlike octopus-folk, dolphin-folk or manta-folk, some of whom are shown to be at least antagonistic in the animated series), so it's possible that enemy tribes will sometimes eat each other. Triton naturally assumed that humans are also cannibals who happen to also eat fish, meaning that merfolk are somehow extra delicious.

Atlantica has a magic presence in it that causes anyone there to have an irresistible urge to sing.
This explains Ariel and Ursula randomly bursting into song. Sure, Sebastian performs a musical number ("Kiss the Girl") for Eric and Ariel, but it's implied that the song was written beforehand. This also explains why Sora suddenly enjoys this while in Atlantica.
 * But what about Les Poissons? That's sung by a character who's not only not a resident of Atlantica, but who's also a chef who takes pleasure in chopping up and cooking fish. Interesting theory, though.
 * Though considering that he also told a crab to "Fight like a man" he may just be a Cloudcuckoolander.

The little mermaid was based on The Beatles' Octopus's Garden.
Or at least parts of it, Ariel's cave sound very similar as how The Other Wiki describes an Octopus's Garden, and also both Under The Sea and Octopus's Garden sing about the joy of living under the sea, free from labor and dancing around all the day. Hell, the first line of Octopus's Garden is "I'd like to be under the sea".
 * Jossed. The Disney film was actually based on the Hans Christian Andersen fairy tale of the same title, which was written in 1836.
 * Although the soundtrack album of the sequel does include that song, sung by Sebastian!

Vanessa was the original form of Ursula, was Triton's sister, but her use of dark magic corrupted her and turned her into an octopod.
. If you really think about it, it wouldn't be all that implausible. Let's say, for the sake of argument, that Ursula was in fact Triton's sister like some sources say. Well, why is she half octopus instead of a regular mermaid like her brother? Her dark magic corrupted her. Before Ursula had eight legs, she had a fin just like Triton. She wanted the power to rule the ocean but couldn't because Triton was on the throne. Because he had more power then her, she looked to darker arts for more power. The best way was to turn the merfolk who came to her for help into those slug things. Every deal she made gave her some sort of edge. While she got magic and the potential to over throw Triton, it corrupted her and turned her into an octopod over time. Triton caught on quickly and promptly banished her. Since she's been isolated for a good twenty some years (if Triton's daughter's ages by the first movie are any indication) then she's been collecting more victims so by the time of the movie, Ursula has enough power to overthrow Triton. She just waited for the perfect opportunity. Also, look at Vanessa's appearance: she may not look a lot like Ariel, but she looks like some of Ariel's sisters. Compare the shape of her face to Aquata and Andrina and her hair to Attina and Arista. Quite the similarities between them, right? It would make sense that an aunt would be similar to her nieces in some ways, wouldn't it?
 * THIS SHOULD BE CANON!!!!
 * Which also begs to question about Morgana's role. My speculation is she IS in fact Ursula's sister and Triton's too. To Elaborate: Morgana and her mother were both regular mermaids once upon a time. She had Morgana from a previous marriage and when her spouse died by whatever means she married Triton's father. This means Triton and Ursula could have been the half-siblings of Morgana. As the second movie tells us, she was her mother's un-favorite. She wasn't the King's daughter so he probably didn't care. With Ursula her mother's favorite and she probably having nothing in common with Triton Morgana grew resentful and lonely. When Ursula tried to use her dark magic to gain the throne, perhaps she got the idea from Morgana who wanted the power to impress her mother and gain her favor for her own selfish reasons but backfiring.

The young blond merboy in the sequel that Melody befriends is the son of Urchin.
The boy has blond hair and the same nose as Urchin. He's probably a little older then Melody and may take more after his mother. Also, his tale is the same color as Urchin's. Their friendship may also be a nod to how Urchin and Ariel met when they were younger.

Trident had a harem.

 * Basically, King Trident had many wives and sired daughters from each one, though only one (Ariel's mother) was officially considered queen. This explains why all his daughters look so different from one another. While for the most part, he treats his daughters equally, he favors Ariel to some extent because she's the daughter of the official queen, his favorite wife.