To get the Hunted cheat, you'll need to have around 50 Abstergo challenges completed. The Hunted cheat makes it so that you always have a max Wanted level in the sea. I recommend having the Morrigan fully upgraded to make this easier, though it's not necessary.
1. Turn on the Hunted cheat.
2. Fast travel to the most southern part of the North Atlantic and start destroying ships. Every enemy ship counts, even the little gunboats, so just start sinking everyone you see.
3. Once you reach 10, the achievement will pop.
I didn't board any enemy ships during this, so I don't know if boarded ships will count, but to be on the safe side, just sink every enemy ship you see.