This time you'll have plenty to do - you have to kill the ship's captain and six member of a certain gang. Besides, you have to retrieve some photos. Let's get to work, then.
Hide between the crates to the right (looking from the starting position) and wait for the sailor to get near. Sedate him and hide his body somewhere around here. Pick the lock in the door to the machine room and find your first target on your map. Wait until he's looking down the machinery over the railing and push him into it.
Find the stairs around the stern and get to the 4th Deck. Along the way you'll see another gang member, but you can't stealth-kill him right now. You'll end up in a restaurant. Go forward to the next room and leave your guns in the toilet (1). Go on to reach a bar. Before you'll be able to proceed, one of the guards will scan you for weaponry. Go forth, and in a room (2) you'll find a waiter's uniform. There's another gangster wandering around the stern. He's easy to deal with, as he's alone and near the railing. Just use your fiber wire and dispose of the body by throwing it to the river. Go to the kitchen (3) and grab a cake. Go to the toilet with it and drop in on the floor, then inject some poison into it. Take the stairs to get to the top deck and let another sailor scan you. Enter the captain's quarters and leave the cake on the table.
When you see that the captain's dropped dead because of the poison, get back to his room and enter the bedroom. There's a safe in there, take the photos out of it. You'll find two or three gang member on this deck. You deal with each of them in the same way - just wait until no one is watching and fiber-wire them, then dispose of the body. It's not difficult, as there're not many witnesses who scoop around.
You're left with one target to take down, the guy who passed you when you walked to the restaurant. You'll take care of him the same way as you did with the previous ones - now he'll be walking along the railing of the ship. When you're done, take your weaponry and your suit, then leave using the boat hanging at the stern.
HINT: On the second deck there's a cool weapon to pick up, together with a suitcase you can hide it in. It's marked on the map above (*).