It's a really fun game but perhaps not as good on the PSP.
The combat mode is good but not quite. Ship combat can be frustrating at times but in average is really good. If you get close enough to a ship you'll be able to fight sword to sword. Sword to sword fighting is really awesome but once you've done it for a certain amount of times it will seem repetitive and boring.
The graphics are good for a PSP game so you'll have no grievances about it. The sound is what sucks the most on this game. Sometimes you would be sailing and if you're not near a port or village the game will have no sound at all which makes the game look empty.
Overall, this game is worth playing specially because is not that expensive. Sure it has some faults but it is still an "ok" game. You'll get bored of this game at some point but it is still worth having in your collection.