It depends upon what you want your pirate to be:
Barbarian: Ranks in Climb, Swim, Jump, Intimidate, Profession (sailor) and Survival; Feats like Power Attack, Cleave, and Kiai Shout; this will be the "Do what I tell you or you'll end up as Chum" kind of pirate.
Bard: Ranks in Climb, Swim, Intimidate, Bluff, Diplomacy, Profession (sailor), Knowledge (geography), Sense Motive; feats like Leadership, Weapon Finesse, and Intimidating Strike; this will be the kind of pirate who uses his bard song to rally in battle, and guile to keep his sailors in line and in plunder.
Druid: Ranks in Knowledge (nature), Survival, Spot, Profession (sailor); feats like Elemental Mastery, Energy Substitution; this pirate is the wrath of the wave and storm, not seeking riches but revenge for past offences.
Fighter: Ranks in Climb, Swim, Jump, Intimidate, and Profession (sailor); Feats like Dodge, Mobility and Spring Attack; this will be the pirate who holds onto his job by his ability to out-duel rivals.
Monk: Ok, I can't see a monk pirate. Sue me.
And aside from the Paladin, who might make a good pirate-hunter, the rest of the classes follow suit; pirating is not a class: it's a way of life. It's a skil-set and flavor. Wizards will harness the energy of Wind and Wave; Clerics will prosecute the will of their watery deity; Rogues will see opportunity for plunder, and mobility on thier ships; Water-borne Rangers will have the same regard for the sea that most rangers have for their forests.
So any class or PrC can turn pirate, just as any class can be an effective brigand on a kingdom's highways.
Barbarian: Ranks in Climb, Swim, Jump, Intimidate, Profession (sailor) and Survival; Feats like Power Attack, Cleave, and Kiai Shout; this will be the "Do what I tell you or you'll end up as Chum" kind of pirate.
Bard: Ranks in Climb, Swim, Intimidate, Bluff, Diplomacy, Profession (sailor), Knowledge (geography), Sense Motive; feats like Leadership, Weapon Finesse, and Intimidating Strike; this will be the kind of pirate who uses his bard song to rally in battle, and guile to keep his sailors in line and in plunder.
Druid: Ranks in Knowledge (nature), Survival, Spot, Profession (sailor); feats like Elemental Mastery, Energy Substitution; this pirate is the wrath of the wave and storm, not seeking riches but revenge for past offences.
Fighter: Ranks in Climb, Swim, Jump, Intimidate, and Profession (sailor); Feats like Dodge, Mobility and Spring Attack; this will be the pirate who holds onto his job by his ability to out-duel rivals.
Monk: Ok, I can't see a monk pirate. Sue me.

And aside from the Paladin, who might make a good pirate-hunter, the rest of the classes follow suit; pirating is not a class: it's a way of life. It's a skil-set and flavor. Wizards will harness the energy of Wind and Wave; Clerics will prosecute the will of their watery deity; Rogues will see opportunity for plunder, and mobility on thier ships; Water-borne Rangers will have the same regard for the sea that most rangers have for their forests.
So any class or PrC can turn pirate, just as any class can be an effective brigand on a kingdom's highways.