I got this achievement after the opening cinematic. You swim to the shores of Cape Bonavista. Instead of chasing the assassin (except at the point where you have to) take your time in each section and find all the treasures (lots of them, I ended up with about $5000) , bottle, animus fragment, Mayan statue, shanty and treasure map. Make sure your maps is clear. Do all that and the achievement should pop. Simple. Took maybe 15-20 minutes but it was the start of the game and I didn't have anything else to do yet anyway lol
Cheers