Before you can eliminate the main mission target, first you must get onto the deck of the yacht. More precisely, you must gain access to the second level where Kalvin Ritter can be found. One of the ways of doing that is using the southern gate (M1,4). You will start in position (M1,1). Collect the lockpick and coins. Get close to the gatehouse (M1,2) and throw the coin in order to make the guard change his position. Quickly sneak up behind him into the gatehouse.
Once you reach the gatehouse (M1,2), wait in hiding for the guard to go back to his post. Now you can continue your journey towards the gate or you can stun the guard, hide his body in the closet and wear the yacht security disguise. This disguise will make it much easier for you to move through levels 0 and 1. In order to open the gate you need the lockpick. Picking the lock will take Agent 47 few seconds.
Important - If you obtained yacht security disguise, then you can ignore the gate and go to the main bridge that leads to the yacht (M1,3). This will allow you to reach level 1 faster. However, watch out, as at least one of the nearby guards can recognize Agent 47 despite the guard clothes he has on.
Another way to infiltrate the yacht is by using the car platform in the northern part of the map (M1,5). If you pick this option, then you should have the security guard clothes or mechanic disguise. The latter can be found in the northwestern corner of the map (M1,1). It is in mechanic's line of sight, so you must lure him someplace else (for example by using the coin). You can also stun him and hide his body in the toilet. Once you reach the yacht deck, you must watch out for the new mechanics as one of them is very suspicious and he can recognize Agent 47 whenever he gets into his sight.
If you start the exploration of the yacht on level 0, then you can take interest in one of the eastern rooms in which you can find a very valuable yacht crew disguise (M1,2). Wearing these clothes will allow you to freely travel through the whole yacht - the only person you will have to be afraid of will be Ritter.
You can reach level 1 by using any stairs. The only exception is situation when you don't want to make Agent 47 wear new disguise. In that case you can disorient the guard near the generator (M1,6) and then use the scaffolding to climb on level 1.
You can travel freely through level 1, but you won't be allowed to level 2 unless you use crew member disguise. You can obtain such disguise (one of the additional opportunities for obtaining it is attacking the chef in (M1,9)) or find new scaffolding in the eastern part of the yacht (M1,8) and use it to climb.
If you reached level 2 in a crew member disguise, then you must only avoid being detected by Ritter. Otherwise you must avoid every guard on the yacht.
An alternative method of infiltrating the yacht is pretending to be Mister Norfolk with whom Ritter is supposed to meet. You can find him at the car near the starting location (M1,14). Norfolk is in the line of sight of at least two guards. It means that you must lure him to the unguarded western buildings. The best way to do it is by using the coins collected from the starting location or by using other throwing objects. Throw one coin after another while waiting for Norfolk to walk towards them. Attack him from surprise only after he moves significantly away from other people. Once he is knocked down, put the Terry Norfolk disguise on and hide his body.