It's quite enjoyable to have a whole campaign where everyone is a pirate or outlaw. In pirate campaigns, all the characters are raiders of the seas; in outlaw campaigns, everyone is a lawless warrior. In both campaigns, the bands of characters prey on the weak (but wealthy) and may have important goals to achieve other than just taking money away from people who have too much of it.
The DM should determine and then make it clear from the outset whether this is a "good guy" or "bad guy" campaign.
Good-Guy Outlaws and Pirates[]
If it's a "good guy" campaign, the player-characters are heroes... but misunderstood.
They may have been framed for crimes they did not commit, and were forced to flee the authorities (it's hard to prove your innocence when you're swinging from a gibbet). They may be enemies of the new ruling power (if a wise old ruler dies and is replaced by an oppressive and unfair new ruler, that's ample justification to embark on a life of outlawry... in the game and the movies, anyway).
In such a campaign, the characters are wanted by the law, but it's the law that's wrong, and the heroes treat their captives and victims with golden-rule ethics.
In other words, they'll capture innocents and take their money and goods... but they'll offer no insult to victims who deserve none, will tend to release such prisoners unharmed. Inevitably, some of these former prisoners will be re-introduced in the story in an upper-hand position, and may be able or willing to help the unlawful heroes when they're in a bad position.
On the other hand, victims who are their true enemies (wicked representatives of the evil rulers, personal enemies, tax collectors, competing pirates and outlaws with no scruples, and self-centered money-grubbers of any sort) tend to be humiliated and embarrassed while they are prisoners of the PCs. Unless they behave very stupidly and attack the PCs, they, too, are likely to be released unharmed; if they do attack, they tend to be battled in single combat, and usually are killed. Enemy prisoners, if released, also tend to reappear in the story down the line... usually at a time when they can do a lot of harm to the player-characters.
The main goal of this sort of campaign is restoring the old status quo. If the characters used to be law-abiding citizens and are now wanted by the law for the wrong reasons, their eventual goal is to prove their innocence. If the land used to be ruled by a wise ruler who is now imprisoned or dead, the characters' goal is to release him from prison or find and crown his wise true heir.
Often, the heroes' force of men (pirate ship or outlaw band) will prove useful to someone in a position of power (like the rescued ruler or another powerful noble), and that personage will issue them pardons and commissions into his army or navy just before the slam-bang climax of the campaign or mini-series.
Bad-Guy Outlaws and Pirates[]
If it's a "bad guy" campaign, the DM and players will have to define the campaign goal. It may be nothing more than the acquisition of treasure over years of outlawry. It may be considerably more specific, such as the finding of a treasure buried by a famous long-dead pirate, or the execution of a brilliant robbery plan.
The bad-guy campaign is mostly suitable to characters of neutral or evil orientation, and to players who just want to play in a nasty fashion for a while. Their characters don't have the dainty ethics of the good-guy pirates and outlaws. No, they rob everyone, from other villains to virtuous maidens to pious clergymen. Prisoners in their hands won't know what to expect; they may get not-too-uncomfortable imprisonment for ransoming purposes, or brutal mistreatment, or death, or worse than death, however the player-characters feel. Further, the PCs' feelings, and their treatment of prisoners, may change from day to day.
This is a dangerous environment and campaign. Since the PCs are as scummy as their worst enemies, there probably won't be any DM sympathy to help keep them alive in bad situations. The PCs will be competing on equal terms with the nastiest of villains, and you can expect a high body count among PCs and NPCs alike.
On the other hand, when you're in a destructive mood, it can be a lot of fun.