Locations on the map
Starting area
There are three items on a table nearby - remote explosives, a lockpick and coins.
You'll find a rifle and a shotgun inside the left building - you'll need them only if you want to finish the mission by playing aggressively.
Guard post
You can distract the guard using a coin and then sneak past him or wait for him to return to his post and subdue him (yacht security disguise).
Gangway
You can use it if you're wearing a disguise.
You can open the gate using a lockpick found near the starting area (blue 1 on the map).
Entrance for the cars
You can use this passageway to infiltrate the yacht, however you may be easily spotted if you're not wearing a disguise.
Scaffolding
You can subdue or distract the guard by the turning off the generator and then climb the scaffolding to reach level 1.
Fuse box
You can sabotage the fuse box so that a mechanic shows up to investigate. Subdue him to get a mechanic disguise.
There's also a fire alarm switch here.
Scaffolding
You can use the scaffolding to reach level 2. This is a good idea if your current disguise prevents you from using the stairs.
Galley
A yacht crew member is here. You can subdue him and obtain his disguise.
Bar
You can use the bar to blend in assuming you're wearing a yacht crew disguise. This will allow to wait for things to develop while remaining hidden.
Window
You can use this passageway to access Ritter's quarters.
Motor boat
You can use it to leave the mission area after killing Ritter.
Helicopter
You can use it to leave the mission area after killing Ritter.
Mr. Norfolk
He's standing close to a gangway leading to the yacht. You can move him away from the yacht by using coins or other throwable objects and then subdue him and put on his suit. This will unlock full access to the yacht and you'll be able to get to Ritter's quarters after having a short conversation with him.
Important items on the map
Mechanic disguise
An unusued disguise is on a crate. It's in the line of sight of a mechanic and you can distract him or subdue him.
Yacht crew disguise
The clothes are on the chair.
Rat poison
You can use it to poison Ritter on level 2 (red 2 on the map).
Crowbar
You can use the crowbar to drop a life raft (red 4 on the map) on Ritter in order to kill him in a staged accident.
Suggested assassination spots
Ritter's quarters
You have a free choice on how you want to kill Ritter and you can do it when he's here standing next to a desk. You can access the quarters by climbing through a window (blue 11 on the map).
Once you've killed Ritter you can hide his body in the bedroom.
Wine glass
Go to the table with a woman sitting next to it and pour the rat poison into the wine (use the yacht crew disguise). The poison can be obtained on level 0 (green 3 on the map).
After Ritter has tasted wine he will go to the bathroom (red 3 on the map) and that's where you can kill him.
Toilet
Ritter will show up here after drinking poisoned wine (red 2 on the map). You can follow him to the toilet or you can go there earlier and hide in the locker until he shows up. Kill him using your favorite method and hide his body in the locker.
Life raft
You can drop the life raft on Ritter after he stops by the table on level 2 (red 2 on the map). This can be done only if you have a crowbar (green 4 on the map) or explosive materials. You should also eliminate a nearby guard before staging the accident.