Well, the way the story is set up, the mermaid has never been allowed above the water and is therefore drawn to the surface the moment she turns fifteen and is old enough. She spies the prince and instantly feels that she is in love with him, so much that when he nearly drowns, she rescues him at great personal length. Because she cannot go ashore and he does not wake up until the people she discreetly leaves him with revive him, he never sees her at all. Therefore, the prince would've had no reason to go under water. He had no idea who she was when she did eventually take human form and go to live with him in the castle temporarily. But, theoretically, the story could've been written a thousand different ways, and, frankly, I've no idea why he wrote it the way that he did.
There is definitely an element of women trying to please men, to fit in to their world, but at least the woman did get to be the heroine. Were the prince the one to fall in love and chase after the mermaid, he would've just ended up being the self-sacrificial hero in the story, and then someone would be pinpointing that.
(On a separate note, I'm not exactly sure what you mean by her needing her father's blessing to become fully human, as she actually dies at the end and then becomes a fairy, unless you are specifically referencing the Disney movie, and, in that case, I have been referencing the original tales the entire time.)
Anyway, if you analyze it without bias, a lot of the female 'victim' characters do have merits of their own. For example, Cinderella never gave up faith that someday her life would get better. Belle from "Beauty and the Beast" is both courageous and compassionate, willingly going to the Beast's castle to save her father and eventually finding it in her heart to look past the Beast's ugly exterior (even if it takes his dying for her to see it first). I think that more often women end up being the psychological heroines and it's the men that triumph with their brute strength. It definitely relies on stereotypical sex roles, but at the same time, it's not like women are made out not to have any virtues at all.
It's all how you look at it.