Predator
The Predator is a Police assault boat that a player can use in all but one of the Grand Theft Auto III era games and Grand Theft Auto IV. It is a special boat on the games because it has built-in .50 caliber machine guns on both sides of the boat, thus giving the ability to destroy any other boat directly in front of the Predator. This boat appears if the player gets a wanted level on the water. The Predator unfortunately does not have Vigilante Missions associated with it, and this cannot be done on this vehicle.
In Grand Theft Auto: San Andreas, an officer is on the roof of the boat shooting CJ if the player has at least two stars Wanted Level. In GTA IV, the officer is on the back and will shoot no matter what Niko's wanted level is. It is possible that they'll be more aggressive if the wanted level is higher. Also, it appears that the machine guns are either inaccessible to the player or are non-existant in GTA IV.
Locations
GTA III
At the docks next to Asuka Kasen's apartment
GTA Vice City
Below the Links Bridge, next to Leaf Links