Option 1: Using Knowledge Devotion to Compensate for Medium BAB
You could play a medium base attack bonus class and compensate for your BAB with the Knowledge Devotion feat (Complete Champion, p. 60), which would allow you to gain an insight bonus to attack and damage rolls by making a knowledge check. In most games, this would be unreliable unless you put points into the six knowledge skills for identifying monsters. However, because you are playing a pirate campaign, I suspect most of your enemies will be humanoids, which fall under knowledge (local); sea creatures, which fall under knowledge (nature), or if you have a generous DM, profession (sailor); sirens, which fall under knowledge (nature); and ghost pirates, which fall under knowledge (religion).
Because you're interested in maximizing your sneak attack damage, I would recommend playing a Rogue, which gets sneak attack dice faster than any other class. You could dip to gain additional dice, but since your game calls for a less optimized character, that should be unnecessary. If the rest of your group is suspicious of optimization, it might also create unnecessary tension.
Since your background suggests a character with wilderness experience, you could use the Wilderness Rogue, which trades the skills Appraise, Diplomacy, Decipher Script, Forgery and Gather Information for Knowledge (geography), Knowledge (nature), Ride, and Survival.
Assuming an intelligence of 14, you would have 10 skill points per level. To represent your background, consider Hide, Move Silently, Spot, Listen, Knowledge (nature) and Survival. This would make you a great lookout and a vital addition on any excursions to treasure islands.
To represent someone skilled at fighting humanoids and familiar with port towns, you could add Knowledge (local). Assuming you maxed all of the above skills, which might not be necessary, this would leave you with three other points each level. For mobility, you could put them into Climb, Balance, Jump, Tumble or Profession (sailor). Alternately, you could dedicate them to Intimidate, Bluff, and Sense Motive so you have something to when the enemy vessel waves the flag of truce.
So what about combat? Here's where your feats come in. The only feat that is set in stone is Knowledge Devotion, as it is what allows you to compensate for your low base attack bonus.
If you have the dexterity or you're willing to spend time as a ranger, you could take the Two-Weapon Fighting tree, which is generally considered optimal for a damage-focused rogue. Your feats would probably be Weapon Finesse (1), Two-Weapon Fighting (3), Knowledge Devotion (6), Improved Two-Weapon Fighting (9), Craven (12), Staggering Strike (15), and Greater Two Weapon Fighting (18).
If you don't have the dexterity for the Two-Weapon Fighting chain, consider focusing on Staggering Strike to disable key opponents. In that case, your feats might be Craven (1), Knowledge Devotion (3), Power Attack (6), Staggering Strike (9), and Brutal Strike (12).
Option Two: The Real Sneak Attack Fighter
If you're comfortable sacrificing skill points, consider Rogue 4/Swashbuckler 16 with the Daring Outlaw feat (Complete Scoundrel, p. 76), which allows your Swashbuckler levels to stack with your rogue levels for determining your Sneak Attack dice. You only need two dice and the 2nd level Swashbuckler feature Grace to take the feat, so you could get it as early as level 6. Swashbuckler only gains 4 skill points a level, but it has full BAB, good fortitude saves, a dodge bonus to AC against one opponent, and a feature that allows you to add your intelligence bonus to damage rolls with light weapons or weapons that can be used with Weapon Finesse. Assuming your intelligence modifier is above 3 or 4, that is as good as gaining an extra sneak attack die.
If you decide to take the Two-Weapon Fighting chain, consider swapping out the Swashbuckler's dodge bonus for Shield of Blades (Player's Handbook II, p. 63), which grants you a shield bonus to AC whenever you make a full-attack while wielding two light weapons. It is less reliable than Dodge, but it applies to all opponents.
Option 3: A Swift and Deadly Warrior
You could also play a Swift Hunter: a Scout 4/Ranger 16 with the Swift Hunter feat (Complete Adventurer, p. 81), which allows your ranger and scout levels to stack for determining your Skirmish dice. To make skirmish activate, you would take the Travel Devotion feat (Complete Champion, 62), which would let you move your speed as a swift action for 10 rounds. If your GM likes having multiple encounters each day, you could take the feat a second or third time to gain additional uses.
Travel Devotion doesn't negate attacks of opportunity, but that's fine because you have access to Tumble. Scouts get the skill by default, and Rangers can trade Ride for Tumble using the Skilled City dweller option in the Cityscape Web enhancement.
Because rangers can cast spells, I suspect the vanilla version would be banned in your game. Fortunately, you have a way around that: the Champion of the Wild alternate class feature (Complete Champion, p. 50), which trades your spells for bonus feats at levels 4, 8, 11 and 14. You have to select the feats from a small list, but most of the options are decent, and they include pirate-appropriate options like Combat Expertise, Improved Trip, Improved Disarm, and Blind Fight.
Rangers also gain an animal companion, which appears to be at odds with your character concept. There are two alternate class features that get rid of the animal companion: Distracting Attack (Player's Handbook II, page 55), which causes your attacks to grant flanking bonuses to your allies, and Solitary Hunter (Dragon Magazine 347, page 97), which allows you to apply your favored enemy bonus to attack and damage rolls, not just damage rolls.
But you're a pirate, sir! No one in your profession is complete without a peg leg or a parrot, and between the two, I'd take a parrot. But don't settle for a run-of-the-mill bird from the local shop or an animal companion so far behind the power curve it collapses in even the mildest storm. Instead, use the Urban Companion alternate class feature from the Cityscape Web enhancement to gain a familiar.
"Thaliak, you're mad!" cry the millions of wizards who have lost their familiar to a stray fireball. Do not worry, friend. Rangers are much better at finding good familiars than those week, pale-skinned wizards who spend their lives in a library and those pretty sorcerers who think everything should come naturally. Your familiar will inherit 75 percent of your hit points instead of half and benefit from your two good saves, and if it somehow dies, you'll fight on without any loss of experience. After all, it only takes you a day to find a new one.
But why get a familiar? Because they use your skill ranks! With a seagull flying high above, no enemy ship will be able to surprise your crew, for you will get two Spot checks instead of one! If birds aren't your style, try a rat. As long as you have thematically-appropriate ranks in stealth, your little friend will be able to scope out enemy strongholds, search captured ships for hidden compartments, and spy on the first mate or anyone else you want to take down.
But wait, there's more! Your remarkably intelligence rat will help your resist scurvy by increasing your fortitude, a one-feat value! And if you keep him on your shoulder, you'll get another feat free! It boosts the two most important skills in the game, so I don't need any ranks in appraise to tell you that it's worth 19 gold and 99 copper!
Still not convinced? Consider this. If you're ever assigned guard duty, you can close your eyes; your rat or bird can watch the prisoner while you dream of wine and women.
In addition to this amazing companion, the Swift Hunter offers 6 skill points a level, two good saves, 19/20 BAB, four bonus feats, and three Combat Style feats. Since you'll get bonus damage on most of your attacks, Two-Weapon Fighting is the style of choice. And unlike the poor Daring Outlaw described previously, you don't need to meet the prerequisites! That means you can embrace your half-orc heritage and grow up big and strong.
But if you dislike ripping your foes to shreds in a rage-fueled fury. you have other options. The archery feats will let you clear that hapless merchant's deck before you board. Alternately, you could take the Strong Arm combat style (Dragon Magazine 326, page 97) to gain the ever-popular Power Attack alongside the loot-destroying but thematically-appropriate Improved Sunder, and the wanna-be mook slayer Great Cleave. Alternatively, if you want to throw the dagger inside your boot or the axe you've been carrying since your days in the wild, you could gain Quick Draw, Point Blank Shot, and Far Shot.
If I've sold you on playing a Swift Hunter, you might want to consult the Swift Hunter's Handbook. If you're still interested in prestige classes, I would suggest looking at the Sneak Attack section of Lists of Stuff to find ones that boost sneak attack. The Scarlet Corsair and Dread Pirate look thematically appropriate, but I don't have the high-level experience to evaluate their power.
What I will say is this: As you look at different options, keep in mind that much of the optimization advice online assumes a normal game, where melee characters must earn their place alongside mighty wizards, clerics and druids. In a group of brutes, you probably don't need to push your build to its limits. In fact, because all of you are likely to focus on dealing damage, you might need to hold yourself back to make sure you don't overshadow other players with less interest in optimization.
