Triton made escape a necessity. Once someone goes to the point of destroying your possessions in a violent rampage, there is no staying and sticking it out, there’s no safety. (And Ariel, even in Ursula’s lair, gave Triton more thought than he deserved at the time.) Nowhere in the ocean she could go and be safe. Everyone’s always ‘why don’t they just leave :|’ in abusive situations until the leaving is not something they find 100% worthy of approval.
Ursula made it about Eric. She didn’t have to. Ariel had to get out from under Triton’s thumb, it could have been literally anything. Ursula took advantage of a desperate victim for her own agenda. Realistic predatory behavior toward a vulnerable person.
There’s always the ‘Eric didn’t want her until she was silent and meek’ criticism - FIRST OF ALL he started out looking for a woman who wasn’t silent, and second of all what part of the carriage driving bit (or any of her other actions on land) is meek, exactly?
People above have noted the queer subtext. Now, on the subject of Ariel being willing to leave her family, aside from the baseline ‘this is an abusive environment and she was not safe there’ angle I already mentioned, consider: Ariel’s father made it clear he would stop at nothing to crush and tear down who she was and replace it with what he wanted her to be. Now - what demographic might that resonate with? And given Ashman’s involvement, do you think that was a coincidence?