Raise your hand if you've ever seen any of these in a PotC fanfic:
The character who constantly whines and complains about how horrible her life is, even when she should be focusing on other things.
The allegedly badass character who can't take care of herself, falls to pieces whenever her love interest isn't around, and constantly needs other people to rescue her.
The character with a tragic past whose love interest cures her of any psychological damage.
The character who hates pirates but eventually falls in love with one.
The woman who hates men and thinks anything with a penis is evil, until she meets her love interest.
The pirate captain without a good reason for being captain or going to sea in the first place.
The noblewoman who wants nothing more than to escape her restricted life, run away to sea, and become a pirate.
The barmaid or hooker who spends her days longing for a handsome pirate captain to sweep her off her feet and carry her away to a life of adventure.
They're all cliched, boring, and predictable, right? Right! So don't make your OC one of them. Nobody wants to read something they've already seen a hundred times.
Your Character's Backstory
Backstory is what happened to a character before the story started. Mary Sues tend to have terrible and tragic backstories that provide them with constant sources of angst in the present. If your character has experienced any kind of trauma, make sure she reacts to it realistically and doesn't use it as an excuse for cheap sympathy.
Where is your character from? Who are her family members, and how does she get along with them? Where does she live now, and how did she come to be there? Why does she have the job she has now? Why does she believe what she believes? Has she done anything she's particularly ashamed of or embarrassed about? Why does she behave the way she does?
Whatever the answers to those questions are, don't dump all the backstory on your readers' heads before they've had a chance to get to know your character. Instead, give it to them in bits and pieces. Your readers will appreciate that. And if the information is not necessary for plot or character development, leave it out entirely.
Your Character's Personality
No matter what fandom they're in, Mary Sues all have the exact same personality: rude, self-centered, and whiny. OCs who are neither annoying enough to be actual Mary Sues nor developed enough to be well-written characters tend to be copies or female versions of canon characters. Your character should be her own person, and I mean a person your audience can relate to, instead of a blank slate, an inferior version of a canon character, or an unrealistically perfect avatar they're supposed to admire.
Intermission: Some Things You Should Know
Informed attribute: Something a character supposedly has, but that the audience never sees any evidence of. For example, the supposedly compassionate character who doesn't do anything to help others or stop them from being hurt. "Show, don't tell" is the number-one rule of writing for a reason.
Flaw: A personality trait that causes problems for a character. Mary Sues don't have problems, so they have no real flaws. On the other hand, if you want readers to like your character, her personality shouldn't solely consist of flaws. A good character should be a mix of positive and negative traits.
Respect: Something your character should earn, rather than receiving it on a silver platter.
Character development: How a character changes over the course of the story. Starting off special and becoming even more special does not count.
Straw feminist: A very bad thing to be. Take a long, hard look at every female character in PotC. Do they go around proclaiming how superior they are to men all the time? No! They don't need to, and neither does your character.
Feisty and bitchy: Are not synonyms. "Feisty" is actually kind of a backhanded compliment.
Specialness: Not a substitute for personality.
We now return you to your regularly scheduled program. Here are some questions to help you develop your character's personality.
What does your character want more than anything else in the world right now, and why?
Does she prefer interacting with others or being by herself?
What is her greatest fear?
Would you want someone to treat you or your friends the way she treats other people?
What does she like to do in her spare time?
What does she hate to do? What irritates her?
How does she cheer herself up when she's feeling sad?
What could she talk about for hours, given the chance? What topics bore her?
What is she good at? What is she bad at?
If she could have any job she wanted, what would it be? Why does she want that particular job?
What does she think about when she's alone, other than her angsty past or her love interest?
How does she react to being insulted?
What does she not want other people to know about her?
Who would want to spend time with her, and why?
What is her favorite place to be?
If you want to learn more about how to develop your character's personality, the website Springhole has an answer for just about any writing question you could possibly have. I highly recommend it!