What is the theory? Hercules and Ariel are related. Hercules is the son of Zeus. Ariel is the daughter of King Triton. In Greek mythology, Poseidon has a son named Triton. So, King Triton is the son of Poseidon thus making Ariel Hercules' niece (although most people erroneously say they would cousins). There are several problems with this theory.
Firstly, this entire theory is based on something from Greek mythology. However, Disney's Hercules movie isn't accurate to Greek mythology. In fact, the title itself is wrong because Hercules was the Roman version of the character while the Greek version was Heracles. If that isn't convincing enough, here are some other errors: Heracles / Hercules is the son of Hera and not a mortal woman, Hades being a devil figure, the titans are elementals, five muses as opposed to nine and Narcissist being a god among other mistakes. As such, we shouldn't assume something is canon to Disney's Hercules just because it was in Greek myths.
Thirdly, Triton doesn't seem of divine origin. It established in The Little Mermaid that his powers come his trident and are not innate to him. If he was a god or demigod, why wouldn't he just have these power? Also, Triton resembles the other merfolk than he does the Hercules version of Poseidon, who is a fish person rather than a merman.
So overall, I don't believe in this theory. The entire thing is based on a coincidence that was clearly not the author's intend. Even if you apply death of the author, this theory still falls flat.