HITMAN

You can earn this achievement during the free form testing of the prologue. Mr. Norfolk is the man in the white suit and hat standing outside the ship near the gangway when the level first starts.

The way I got to him was to get onto the ship (use the method of your choice, I did the maintenance worker route), grabbed the rat poison on the lower deck, switched to a uniform of a ship employee (I used the one on the cabin on the lower deck), then went up to the main party deck. Get near the bar area, staying out of the way of anyone who might become suspicious of you. Ritter will have a conversation with the bartender about Norfolk, then head upstairs. At this point, go over and poison the glass of wine on the bar.

Norfolk will eventually come onto the ship and talk to the bartender, picking up the glass and drinking it. The rat poison will make him sick, so he'll run into the bathroom behind the bar. Follow him, take him out, and switch into his clothing (I had to subdue a witness in the bathroom). Once you're dressed as Nofolk, head back to the bar, talk to the guard at the corner who will tell you Ritter is upstairs, head up there and talk to Ritter, then follow him to the cabin. Once in there, eliminate Ritter any way you please (I used the garrotte and hid the body in the closet to get a few other Challenges out of the way), then make your escape by heading back down to the lowest deck and out the front of the boat (you may optionally want to change uniforms again, although it's not necessary depending on how well you sneak past the guards).

I believe the achievement will not unlock until you finish the level.