On Mission 04: Wetwork - eliminate all hostiles without being detected. You can retrieve a silenced pistol from the first hostile you meet on the pier. This gives you the ability to kill under the waterline and remain undetected. Try your best to avoid line of sight and ensure enemies aren't facing each other when killed. You can also abuse the checkpoint system to keep reloading if you get detected. Just keep trying to take down each group without being seen. As for the boats - I left those till last and just killed the passengers as quick as possible before being seen. The best indication of being spotted is a red diamond on the compass, shouting or being shot at (obviously).