The player as Trevor needs to steal the submersible, this is located hanging from a freighter in the Los Santos Port.
Retrieve the sub
Get to the heist set up icon, playing as Trevor, to start this mission. You will need to steal the submarine that is suspended on a crane from a freighter.
Board the freighter and go to the control panel on board to release the sub into the water.
Tip! To get the gold medal for this mission, you'll need to release the sub without boarding the ship. To do this, get to where the sub is in view, and using a Sniper Rifle, shoot the cables holding the sub.