You have to leave town and avoid the patrol cars. Stay in the back alleys and remember to stay low when there are police nearby. As much as possible, stay out of their line of sight to prevent them from investigating your position. Also, keep on the move when you have the chance. Staying long in one area increases your chance of getting caught. The mission will fail if you get spotted. To avoid confusion, place a waypoint to your destination. The best and fastest route to avoid them is by taking the off-road path beside the expressway. You can get there by heading towards the marina's parking lot. Just continue to your destination and a scene will trigger afterward. The mission will be complete.