The amphorae are not super well hidden. I think most people could find them on their own if they had to, but in case you don't feel like it, here they are. This guide is based on my memory since once you find and break the amphora, it disappears off the map forever. There are a couple of missions where I couldn't remember the amphora location, so if any kind people can comment and fill me in, I will add those to the guide.

Also, once you find and break the amphora, you don't need to finish the rest of the mission. You can immediately quit and it will still count. The missions where you already found the amphora can be seen on the mission select screen with a little amphora icon next to the mission name.

Inside the green enemy base, near the flag.

Also inside the enemy base.

Surprise surprise. It's inside the enemy base.

Very easy to find. Jump down from your starting boat and you're practically there. It's behind some barrels.

Once you break the gate into the enemy fort, turn left.

Go down the stairs to the dock area.

From your starting base cross the river and go left. It should be somewhere in the area with the red circle, maybe next to a washed up boat. Not 100% sure about that.

I think most people probably already have this one. It's right next to the flag.

Inside the enemy base, pretty straightforward.

Inside your base, climb up the stairs and it's on the ramparts

Straight up don't remember this one lol
Inside the purple base, to the right of the sphinx. Thanks to Villou for the info

Another very easy one. Just turn around and it's behind your starting base.

I think it might be on the second floor of the central palace, but I can't remember for sure. Anyone can confirm this?
Second floor of the central building. At the top, turn left then left again around the corner. Thanks to Villou for confirming

It's at the top of the watchtower in the enemy base.

It's behind the house

The following screenshot shows where it should be.