You know where to go, so leave the police station and head towards the marina. Go into the street leading to the cemetery but stop by the gate on the right. It's locked, but take a look at the pavement on its right side. There's a stone lying there (zoom), below which you'll find the gate key. Of course use it to open the gate and enter the narrow street with another gate at its end. Behind it there is a dog about which the customs officer mentioned. You have to lure it somewhere else.
Return to the square and approach the gate on the left (right next to the police station; double zoom). Use the tin opener taken from the yacht to open the tinned meat (save your game). Give the opened tin to the dog through the gate and quickly (before the dog eats it) run to the street where Sylvie was before (square - marina - street on the left - gate - passage - outhouse).
Open the gate and block it with the fence fragment on the middle rail on the right, so that the dog won't be able to get to the part in which Sylvie is (if you won't make it in time and haven't made a save at the right moment, you'll have to go back to the yacht for another tin and repeat the process from the beginning). Knock to the door and talk with Sara, who will tell Sylvie a long and interesting story and afterwards invite her to the hangar (go there), in which she keeps an airship (behind the gate by which you fed the dog). Go back to the count. The man will decide to buy the machine from Sara and will send Sylvie to the yacht to get the money and cheques.
Once on the deck, go to Sain-Germain's cabin, take the golden coin left by the killer on the desk (zoom) and afterwards take a look at the painting above the bed. Move it and you will see a safe. The code, at the count said, is Briggs' year and month of birth, entered from the end (538140). Enter it and accept with #. Take the money and blank cheques.