Head to the waypoint in The Loop, just before reaching Mad Mile, and begin the mission. The man nearby will slowly walk to the news stand and make an exchange for the briefcase. Instead of following the briefcase on the patio, jump the fence and travel behind the news stand while keeping the holder in view. He will hand off the case to another man that will continue to walk away from Aiden, do not follow him. Eventually the man will head down the stairs on the far side of the patio and circle back toward you to get on the boat.
When you objective changes, hop on the motorcycle on the lower patio by Aiden’s position and get a move on. Don’t go at top speed, instead stay in control and follow the boat. You can hit the throttle on straight away parts, but make sure to slow down before hitting any stairs. You do not need to stay close to it, but you want to make sure you stay just within range. Eventually you’ll reach the closed dock where the boat stops.
It is possible to get to the briefcase without a fight by entering from the south side of the marina. Wait for the guard to head up the dock before you enter the restricted zone and then exit the water on the west side of the dock.
Stay close to the metal wall as you follow him and then hop over and take him out as he reaches the top, out of view of the guard on the east side of the slipway. You can now access the briefcase without worry and then escape or take the time to defeat the enemies for some experience.