Having obtained this achievement, I can safely say luck plays a massive part.
This might not be the most complete or effective guide out there however I will run over some basics and share tips I found helpful.
Your main source of chests will be from Gold Hoarder Voyages. These voyages require you to visit a set island and dig for chests. You can get these voyages from any outpost and hold up to 3 at any given time. Voyages can comprise of being just 1 island and 1 chest to multiple islands and multiple chests.
Other chest locations include shipwrecks, caves and strongholds. Not all shipwrecks and caves have chests so I wouldn't go out of your way specifically looking for them, but they can be a welcome bonus if you do come across any. Strongholds are random raid events that can be recognised by a skull shaped cloud above an island fort. Though rewarded greatly, I strongly recommend avoiding these events due to the time needed to complete them and higher risk of player encounters.
Chests that work towards this achievement:
Shipwrecked Seafarer's Chest
Shipwrecked Marauder's Chest
Shipwrecked Castaway's Chest
Shipwrecked Captain's Chest
Chest of Sorrow
Chest of a Thousand Grogs
Chest Possession means on your crews ship or person. Once you have found a chest, placing it anywhere on your ship will register the chest towards the achievement. Chests placed on your ship do not despawn.
Though completely possible using a Sloop ship(1 to 2 players), I recommend using a Galleon(3-4 players). Galleon ships are larger ships and may deter smaller ships engaging you.
Always, Always and ALWAYS turn off your ship lanterns. These light your ship up like a beacon at night and may draw players to you. Often players leave there lights on so you can use this to your advantage.
Having someone remain on the boat using the spyglass is helpful. This not only helps see any ships on the horizon but also means your ship can get out of harms way if another boat gets to close for comfort.
Needless to say, avoid all combat where possible. Sometimes this is unavoidable but making sure all cannons are already loaded with cannonballs saves a bit of time.
Storms aren't your friends. Don't enter them unless necessary and if so, make sure to keep an eye out for flooding on the lower decks. Rain CAN sink your boat if left to build up.
For whatever reason if your ship sinks, your loot and chests will initially float on the water. Immediately return to the location your ship sank and recover chests where possible. These will eventually sink to the bottom however can be retrieved by pressing
on them. They will start to float and rise to the surface again (Don't just assume they despawned, my boat sank and we managed to recover 14 chests which sank to the bottom of the sea).
Hope this helps a little and good luck! Anything I missed or is worth mentioning, please comment so I can update the solution.