There is a lot of mirroring going on in this text. Lawful and chaotic, land and sea, male and female, decay and renewal... it's all about the dichotomies. At some point, somebody may be able to construct a serious interpretation from these observations. For now, I'm just going to let them stand on their own.
Jack Sparrow meets his double, an imposter who is using his name to recruit a crew, in London, England; he discovers that this imposter is actually his former lover, Angelica, who spends the rest of the film acting as his female inversion: headstrong, clever, fatally loyal to her father, and dangerously fickle in her friendships. At the end of the film, Jack maroons her on a tiny island, a fate to which he has often found himself subjected in the past.
Barbossa, the great pirate, has defected from piracy and become a privateer, working for King George II -- a strange second-degree betrayal, the treason of the treacherous. Barbossa has a history of seeking, and sometimes attaining, a sort of twisted dominance over death, which has brought him to the land of the dead and back. In On Stranger Tides, we meet his counterpart, the legendary pirate Blackbeard, a devoted buccaneer who now seeks the same thing that Barbossa has already found and lost again: the control over his own death, which has is destined to come at the hands of a one-legged pirate (the mortal encounter with his own double).
The mermaids are infamous for being sirens who prey on men, using their beauty and sexual allure to drag them into the sea. The pirates invert this relationship, driving the mermaids toward shore and finally catching one of them, whom they capture and drag onto land with them, keeping her imprisoned in a glass coffin.
This image -- the image of dry land as an inversion of the underwater, always interchangable with it -- is repeated in later sequences, especially the image of the grounded ship of Juan Ponce de León, perched on top of a cliff in Whitecap Bay. Looking at it from below, the main characters seem to be walking on the ocean floor, seeing the Spanish ship floating above them.
The Fountain turns out to be a mirror, as well, through which youth can be attained by way of inversion of the aging process. Two identical chalices are filled, and one of them contains a mermaid's tear; the person drinking from this chalice steals the life force of their reflection, the person drinking from the opposite chalice.
And Jack Sparrow, being the film's center of attention, is mirrored in another way, as well: there's a tiny voodoo doll of him floating around, granting his enemies power over him. This doll, appearing in a sequence after the closing credits, prompts Angelica to ponder an important question: is she sympathetic to Jack, her mirror image, who saved her at the expense of her father's life? Or will her cruelty win out?