To destroy Athenian supplies in Megaris in Crumble and Burn in Port of Nisaia in Assassin’s Creed Odyssey walk up to a fire source with a drawn arrow to create a flame arrow. There are war supplies on each of the docks.
All Phokis Guides
All Assassin’s Creed Odyssey Guides

Use a fire arrow to destroy Athenian supplies in Megaris in Crumble and Burn in Port of Nisaia in Assassin’s Creed Odyssey

Use a fire arrow to destroy Athenian supplies in Megaris in Crumble and Burn in Port of Nisaia in Assassin’s Creed Odyssey
The other two war supplies are inside the large buildings in the area.

Use a fire arrow to destroy Athenian supplies in Megaris in Crumble and Burn in Port of Nisaia in Assassin’s Creed Odyssey

Use a fire arrow to destroy Athenian supplies in Megaris in Crumble and Burn in Port of Nisaia in Assassin’s Creed Odyssey