ICA Facility, Freeform Training
Go through the motions of entering the cruise ship until you are onboard. There is an emetic rat poison inside Deck 00 once you board. Once obtained, either knock out the chef or find a cabin crew disguise in one of Deck 00's cabins and run up to the first floor bar. Your target, Ritter, may be near so wait carefully for him to pass or get there before him and then poison the glass on the bar. Since you are in the crew disguise, you won't draw suspicion from the bartender.
Now, Norfolk will take a random amount of time to ascend up onto the ship, but once he does, he will stop at the bar and share a conversation with the bartender while drinking from the glass. He will have stomach issues and retreat to the unoccupied bathroom behind the kitchen. You can preemptively wait here or meet him here, knock him out with the door closed, and then take his disguise. Now, ascend to the next deck, talk to and follow Ritter to the backroom, and kill him in whatever fashion you desire.
You can also try to lure Norfolk elsewhere and get him much sooner but the setup has to be more precise than I can directly offer in written form.