To acquire the sunken artifakt in We’re Treasure Hunters on Keos in Assassin’s Creed Odyssey swim down into the Submerged Minoan Palace. Swim to the back of the palace and down into the lower area.
All Keos Guides
All Assassin’s Creed Odyssey Guides

Swim down to the lower area through the hold in the ground to acquire the sunken artifakt in We’re Treasure Hunters on Keos in Assassin’s Creed Odyssey
The sunken akrifakt is in a chest in the lower area.

Open the chest to acquire the sunken artifakt in We’re Treasure Hunters on Keos in Assassin’s Creed Odyssey