This is one of the setup missions for the Merryweather Heist, Trevor goes to the Port of Los Santos and steals a submarine so that he can use it to steal a nuclear super weapon from Merryweather. Once he steals the submarine, he drives it to the drop off point where Floyd uses a crane to lift it out of the water and onto a truck.
From there, Trevor drives the truck with the submarine to the warehouse to store it until the heist is ready to go.
Retrieve the sub
Drop the sub off at the warehouse
Get out of the truck
Gold Medal Objectives
Time – Complete within 08:30.
No Boarding – Steal the Submersible without boarding the boat.