There are a total of 7 buoys to hit in the water. if you look at the dock at the start, there is one to the left, and one to the right to hit. Now look at the water behind the ship that some targets are on (basically the main ship for the mission). There is one in the water behind the front end of the ship.
The next 2 are in the water roughly behind the middle of the ship, one is closer to the ship and a bit further into the water on the right is the 5th one. Now look to the left of the ship and you'll see some blue cranes. The one closest to the back of the ship (the second one back overall if you count the one right next to the ship), between the beams of it you should see another buoy.
The final one you'll need to look past the back of the ship. You should see a small sailboat sitting idle in the water. This might have just been an issue for me, but I had to zoom out one step from the maximum zoom, next to the ship is a buoy. Shoot it for a small Easter Egg going back to Sapienza in Hitman 2016.
If this was a bit confusing, here is a video: