Not really a guide just some advice
Play in 2 man sloops with someone who also wants to go for this achievement. As soon as you see where the treasures are, go to the closet chest to a ship where you can drop off the chest. Have one player on the boat, while the other players runs on to the island and dig up the chest. Then once you have the chest have the player on the boat harpoon the player on the island carrying the chest. (The player holding the chest should turn their back to the ship so the harpoon grabs both the player and the chest with one hit) Then head right to the drop off ship and cash it in within 5 minutes.
Try to go for small island chests since it's easy to dig up and get back on the ship compared to big islands.
Also communicate where you want to pick up the chest so the person on the ship can sail around the island so they aren't a target.