Breadcrumbs

You have to stay close and just monitor the exchange of the briefcase. Open your Profiler to monitor the targets. You have to follow the targets without being seen. Be careful of the hostile enemies in your area as well. Once the drop off has been made to a speedboat, find the nearest one and pursue to the target. Maintain your distance to avoid alarming your target.

After reaching the marina, you have to locate the briefcase without being seen. Swim to the dock and infiltrate the marina. You can circle around to the south with your speedboat and snipe the guards there. Once clear, you can just walk in to pick up the briefcase. After the conversation with Clara, the mission will be complete. Before you can start the next mission, you have to hack to ctOS control center in the Mad Mile district first.