"Go to Pier 400" is an objective in the story mission Scouting The Port in Grand Theft Auto V
Walkthrough[edit]
Follow the GPS over to Pier 400. Trevor will be grilling Floyd to learn what is going through the port. Floyd doesn't get to the good part until close to the end when he mentions Merryweather Security keeping close tabs on a particular freight ship.
Head on through the gate and go over to the left and park.