Before the level starts, you should add one handgun to your inventory (for hostage/knockout purposes). Once the level starts, head straight up the stairs in front of you and to the right. Head past the member of the gator crew.
Note: Alternately, you can simply push him over the railing. They will find the body, but it will count as an accident. If you do this then you can ignore the section about taking the guards uniform and heading to the engine room at the end of this walkthrough. Instead simply retrace your disguise steps and head to the exit.
Head to the far end and in the door. You will see a couple kissing in the hallway to your left along the main hall (another target there). Walk quickly past them so as to get their attention (and breaking their kissing cycle). Turn around and head to the double doors. Go through them and wait. You will see a servant pass towards the bathroom. Follow him and then sedate/knock him out when he is in the bathroom. Quickly grab the body and pull it further into the bathroom so that the door can close. You can grab his outfit, but you only need his master room key.
Wait for the guard to go into his stall in the bathroom and then leave. Head back to the door you entered the inside of the boat from and wait. A civilian woman will walk to the rail. Bump into her and make her turn to break her wait cycle. One of your targets will also go to the rail. Wait as a guard will be coming out shortly. Wait for the guard to move out of sight and then take the target hostage and knock him out. Throw his body over the rail.
Note: the railing at this section of the boat seems to be glitched, as you can not simply push him over the railing. It is very important to knock him out and then throw him over if you are looking to get only accidents.
Head to room 323 (almost directly across from the bathrooms) and change into the tuxedo and grab the VIP pass. Head up the stairs located in the same section up into the VIP part of the boat. Go past the guards at the top of the stairs and through the set of doors in front of you. Once through the doors immediately take a left turn and go to through the last door on that side. This will be a poker room. The men playing poker should have just left it. Go to the other door in that room and pick it. The guard should have left the next room by the time you finish picking the lock. Go across the room and you will find a new outfit by the closet (1st class server).
Exit through the remaining door in this room. It takes you into the hallway. Head to your right and go through the first door (the kitchen). Go to the cake and pick it up. Leave out the door right near the cake. This takes you outside. Wait for the one guard/target to go to the other side of the boat, then drop the cake and place your chosen gun into it.
Note: You can choose to sedate the cake as well if you like (and still have any syringes) – this will knock out the captain when he tastes it.
Head upstairs and walk to the door by the target/guard with the shotgun. Wait for him to scan you and enter. You will find yourself in a very short hallway. Take the door on the left (this will bring you into the bathroom). Wait for the target gator to leave and then drop the cake and retrieve your gun. Wait patiently and the guard will come back. Sneak up on him and then sedate/knock him out. Leave him there for now.
Go out into the hall and head left into the large games room. Walk to the end and go into the captain's section. Place the cake on the desk and then knock out the captain (if the cake hasn't been tampered with). If the other gator comes in, wait, and when he checks on the captain, take out your gun and grab him. Knock him out as well.
Sneak into the showers and then sedate/knock out the server showering there out. Head to the door opposite the showers (the one with the lovely heart shaped bed). Go to the safe and retrieve the parcel.
Head outside and then sneak up on the server there and then sedate/knock him out. Go inside again and then do the same to the server in the games room.
Exit outside via the bedroom and then sedate/knock out the shotgun gator.
One more knockout and then clean up. Go down the stairs and wait near the stairs (as the area the gator can walk is narrow and thus must pass you). You can then simply shove him over the edge (which somehow brings your disguise closer to being blown) or you can sedate/knock him out.
Now clean up. Drag every target body to the edge of the rails and throw them over.
Note: It does not have to be off the boat as the short rails on the top level seem to be just as lethal. Just make sure that your choice of rails will not land on a lower deck where there is a chance it will get spotted.
Now climb down the trellis located by the kitchen. A guard will come that location on that lower deck. Grab him knock him out and take his clothes. Now head through the section you are in to the stairs down. Head through the crew quarters section and into the engine room. Wait and simply push or sedate/knockout the remaining target and throw him over the engine railings.
Go retrieve your suit and then sneak to your exit. This should give you no kills and seven accidents.