Some players believe that some form of pirates may appear in Grand Theft Auto V, whether they're dead or alive. Regardless of age, no pirates have appeared in this game, or any other games for that matter. The only real evidence for their existence is the ability to use the knife underwater, as it is useless on sharks (for comparison, Trevor can stomp a mountain lion to death, as long, as he is the first one to land a blow).
Aside from that, it's mostly speculation from the large map, the large Pacific Ocean, the hidden treasure and sunken ships out in said ocean, and the existence of plenty of random encounters in the game. There is a similar theory that rival treasure hunters were originally going to be present in the game, potentially taking objects from wrecks before the players take them.
There are also several pirate-themed Easter eggs. For example, in GTA III, there is an advertisement for a play called Pirates in Men's Pants, a parody of the real-life play Pirates of Penzance. In GTA San Andreas, there is also a casino called Pirates in Men's Pants. Its slogan is Prepare for a jolly rogering. This may also be a Rockstar's adult joke, because the casino's name can be abbreviated to PIMP.