Tail the barge without touching the ground, then steal the orders and make your escape.
Assassin's Creed: Unity guide - Sequence 9 Memory 1: Starving Times - Steal the Orders
Both optional challenges here can be tricky. The best advice I can offer regarding the first is to make sure you watch the parkour paths ahead of you rather than the boat; just aim to move along the river and you'll catch it eventually. As for the second, you're either a ghost or you're not - but I'll do my best to guide you.
- Don't touch the water
- Don't trigger any alarms
Go to the Docks
Do yourself a favour and pause the game - go pee, have a glass of water, and wipe the sweat off your palms. The next objective is going to be long. All you have to do to trigger it is reach the objective marker then watch a cutscene.
Tail the Barge
Here we go. Run, assassin. Don't ever bother trying to kill guards and have no fears of alerting them; you should move so fast it's never a problem. The path will take you across the sails of boats, pylons in the water, over docks and even up onto the stonework of the bridge and nearby bank at some point. Do everything necessary not to fall in the water and waste time; remember, free-run-up and free-run-down are your best friends, as they keep Arno on the path you want. You'll know you did alright when a cutscene plays; it may even trigger in mid-air.
Steal the Orders
There are several ways to approach this section of the mission, but since you don't want to alarm any guards, the most important thing is to be cautious and patient. Use Eagle Vision, use cover, and retreat to cover whenever a guard becomes suspicious. If a guard comes to check out a body, wait for him to give up and bugger off before advancing again.
Remember, you can always swim around to the far side of the dock, where just one guard stands between you and your target. I preferred to approach from land, killing off everyone I encountered. In either case, be especially wary of the long-sighted snipers. When you kill any guards on the docks near your target, he and a nearby friend may come and investigate; I killed them both rather than risk an alert, and looted his corpse. There's too much risk of detection while stealing.
Escape the Area
However you got in is the best way out, as you should have killed off every threat that might spot you. When you reach the area border, breathe a sigh of relief - that's your optional challenge in the bag.