Piracy is a deleted activity in Saints Row 2.
The data files for this activity have been found in the files of the PC and Xbox 360 versions of the game, and it seems that this activity involved boarding Cargo Ships to kill guards and steal a number of items, then returning them to your boat, and taking the stolen goods to a drop-off point. On the way to the drop off point, there was ambush points where police in boats and helicopters would appear.
The Miami has a Piracy variant defined in the game files. It is equipped with sirens, so may have been a special police variant. It appears that The Protagonist would have been riding a Shark for this activity.
As the data files for this activity were incomplete, some of the details of this activity are uncertain, although the map co-ordinates for the Cargo ship locations have been found. Some of the data files cross-reference the deleted hijacking activity with the piracy activity, but it is unknown whether Piracy was originally supposed to replace hijacking.