Option 1: Murdering Ritter in his private quarters
Kalvin Ritter's private quarter can be found in the northern part of the second floor of yacht (M1,1). The entrance to it is guarded by a single bodyguard, but you can easily bypass him by entering through the western open window (M1,11). Once you're inside, you must simply wait for Ritter to appear in the quarter and to stand next to the desk. Then you can shoot him or sneak up on him from behind and snap his neck or use the garrote.
An alternative way of reaching Ritter quarter is meeting with him when pretending to be Mister Norfolk (details about that can be found on previous page). Find Ritter on the yacht after wearing Terry Norfolk Disguise (M1,14) and speak with him. Now you can go to his private quarter. The rest is similar - kill him quietly in any way you want.
No matter how you acted, you should hide Ritter's body into the small bedroom in his quarter.
Option 2: Poisoning Ritter
In order to poison Ritter you need Emetic Rat Poison. It can be found in the unguarded area in the northern part of deck 0 (M1,3). After finding the poison you must get to deck 2 and take interest in the bar in the southern part of the area (M1,2). Make sure that you have the yacht crew disguise on and walk towards the table shown on the picture - the one with a sitting woman. You must add the poison to the glass of wine.
After poisoning the drink you must wait for Ritter to reach the bar and drink the poisoned wine. He won't die - instead he will go towards the toilet in the central part of deck 2 (M1,3). You can either follow him or go to the toilet before and hide in the large closet (both methods are effective). Once Ritter reaches the toilet, sneak up on him from behind and drown him in the toilet. Then hide his body in the large closet.
Option 3: Dropping the life raft on Ritter
The life raft can be found on the third level of the yacht (M1,4). You can use it to arrange an accident. However, before you do that, first take care of two things - get rid of the guard patrolling level 3 (surroundings of the helicopter landing zone) and find the crowbar (M1,4).
Position yourself at the western life raft and wait for Ritter to appear directly under it (until he stands near the table with a glass of wine (M1,2) on level 2. Once that happens, use the crowbar to drop the raft and quickly go away from the accident place so that no one recognizes you as the person responsible.
Important - You can use the ICA Remote Explosive found in the starting area (M1,1) for dropping the raft. This allows you to drop the raft while being away. However, the explosion will cause more turmoil among the people on the yacht.