Captain Emma Swan and (her former bounty mark) Regina Mills.
My AU head canon for this:
Emma Swan is a pirate captain of her own ship and with her own crew (consisting of various canon Storybrooke characters). She is notorious for her fierce battle skills both with her fists and sword, her tracking abilities due to her grande knowledge of the sea and her wide-spread connections with other pirates. There’s also the fact that no man has ever managed to sink her ship.
One day, while docked on land for an evening to resupply their ship, a mysterious woman named Cora Mills approaches Emma. She knows about Emma’s extensive skills and wants to recruit her for a bounty job. She offers a hefty amount of gold as a reward to capture her daughter, Regina Mills, who escaped to sea a day before she was to marry a powerful man (Leopold) and bring her back to her mother. Cora says that she’s already recruited many pirates before Emma and none could find her daughter. Emma sees this as a nice challenge and accepts (and the gold reward is the giant cherry on top).
Of course, Emma eventually manages to track down Regina, who puts up a good fight and escape attempt, but is eventually stuffed onto Emma’s ship to be transported back to Cora. They set sail for the long trip back. Cue lots of fighting and hating each other at first, eventually giving in to bickering and bantering. Throw in some handcuffed-together-shared-bunks (‘cause eventually Emma doesn’t want to leave Regina locked up in the jail cell and she can’t exactly risk Regina jumping deck in the middle of the night), more bickering in front of Emma’s crewmates who just find the pair incredibly amusing.
And of course some tender moments when they reveal their pasts to each other and Regina reveals why she ran away from Cora and Leopold in the first place. There’s also moments when they teach each other things: Regina teaches Emma about things she learned in school (since Emma was never able to go) and Emma reciprocates with knowledge of the sea and even gives her a sword fighting lesson. They begin to find that underneath all the fighting and hating and, mostly, the circumstances, they actually genuinely like each other.
This is when indecision begins to rack at Emma: follow through and bring Regina back to her mother and her soon-to-be husband? Or…? But before she can come to a decision, they arrive at their destination and Cora and Leopold take that choice away from her. Regina is quickly whisked away and Cora gives her the gold—a simple transaction just like all her previous jobs. (Except none of this feels simple.)
Cora seems to sense her indecision and tells Emma, “Whatever my daughter may have told you, none of it was real. Regina is excellent at manipulation and she would have done anything, Captain, anything to escape.”
Emma stews with this information, holding off on setting sail again for the night. She eventually comes to the decision that was robbed from her. The next day in the town square where Leopold and Regina are to be married, Emma and her pirate crew crash the ceremony to free Regina. Cue epic music as a huge battle ensues between Leopold’s army (who were standing guard at the wedding) and Emma’s crew. After a long and gruesome struggle, Emma and her people emerge as the victors and they bust Regina out of that town.
And because I am cheesy and want a mushy happy ending for them: Regina decides to stay at sea with Emma and the two eventually manage to admit that they may, possibly, by some chance have feelings for the other. They finally find their happy endings.
With each other.