A Watery Grave
Your first contract is to kill a pirate, Gaston Tussaud. Very seldom are you told why someone needs to be killed and this case is no different. I suppose that, being a pirate, he honked off someone along the line and it has finally caught up with him. Tussaud is aboard the Marie Elena, which is docked in the Waterfront district of the Imperial City. He never leaves the ship, so you'll have to find a way on board to get at him. Valtieri kindly provides you with a suggestion that smuggling yourself on board with some of the cargo might be workable. But he leaves it up to you to work out how the deal will actually go down.
You can try the direct approach, if you want. The first mate, Malvulis, and two crewmen are down on the docks and they will attack you if you try to board the ship. I generally make it a point to get them to attack me and let the guards handle them while I clean up the cutlasses and a few gold pieces afterward. Of course this does kind of shut off the option of having yourself smuggled onboard by crawling into one of the cargo crates near the end of the dock. However, once they are out of the way, you can enter by the front door so the smuggling part becomes unnecessary. If you take the smuggling approach, you'll get out of your crate down in the hold on the lower deck and you'll need to make your way up to the captain's quarters near the top deck.
Alternatively, you can jump from the dock to the balcony at the back of the ship if your Acrobatics is high enough. Or you might drop down onto the balcony from the railing at the rear of the ship (it's tricky, but doable). There is a door back there that will let you directly into the captain's quarters, although you'll need to pick the 5-pin lock to get in. I prefer one of these last two methods since I've got no beef against the crew members that I'd have to get around/through in order to get to the captain by using the smuggling approach.
However you do it, put a big hurt on Tussaud. He's three levels higher than you and has a cutlass. He doesn't leave his cabin, but he does sleep from midnight until six if you want to try this quietly. Quietly won't work completely because as soon as the captain dies, two crewmen will come in to investigate. But if you're quick you can grab the key from Tussaud (or not if you picked the lock to get in), pillage his chest and run out the back door. They won't leave the ship to follow you.
Report back to Valtieri to complete the contract and get your point of Infamy, your payment and a Black Band ring (leveled Fortify Light Armor and Security and Resist Magic).