So without going into too much detail about the system as a whole, here's what I have (tentatively) settled on. For clarity, all (well, nearly all) prestige classes have no attack or hit point gain at levels 1 and 2 and no defense gain at level 3; the math is very flat.
Also- a +1 to initiative is a significant bonus.
Pirate
A pirate is a scourge of the seas. Skilled at seamanship and spreading terror alike, some pirates- called privateers- operate legally, under the cover of a charter issued by one government or another. To become a pirate, you must spend a total of at least 4 months at sea.
PIRATE ADVANCEMENT
When you gain a pirate level, add 3d10% to your Dexterity and 2d10% to your Strength. When this reaches 100%, your ability score increases by one point.
Level --- Attacks --- Defenses --- Hit Points --- Features
1 --- None --- +1 Ref --- None --- Pirate's reflexes, seamanship
2 --- None --- +1 Ref --- None --- Fearsome strike
3 --- +1 melee --- None --- +1d6 --- Seamanship improves
Pirate's Reflexes: You get a +1 bonus to initiative.
Seamanship: You get a +1 bonus to skill checks when you are at sea, in the water or on board a ship. At 3rd level, this improves to a +2 bonus.
Fearsome Strike (recharges on a 15+): As a standard action, you make a melee attack vs. Will. If you hit, in addition to normal damage, the target moves half its speed away from you as a free action, triggering opportunity attacks normally.