It is far from a bad film...unless you are the kind of person that just can't stand a fun, pulp adventure story.
Basically all this movie is is a pulp novel turned into a movie.
Had it been made in the Golden Age of Hollywood right up to the 1980s it would have been a hit. Had it been made in the 1990s it would have been a modest success...
But it was made in 2008 and the tastes of the world had changed. 9-11 had come and gone and people lost that childlike innocence that is needed to enjoy films like this. The Computer Age dawned and everyone wanted nothing but over-the-top special effects.
So fun adventures were shoved to the side and the critics--that still claim to love fun adventures like Captain Blood--turned their noses up at anyone that even hinted towards enjoying them.
What you have is a treasure standing in for the MacGuffin, a roguish hero, a little bit of a love triangle, and the open seas.
It is classic Hollywood. It is also, unfortunately, they kind of movie we need today and the kind that Hollywood is most reluctant to make.