There are six locations on the shoreline at the east of the map. All you have to do is find the bottles and read the massages. That places each time a waypoint on the water. Now you have to swim and dive to the locations to find a steamer trunk. Inside you'll find stuff like ammo, weapons, mines, grenades, stimpacks etc. Loot it and the quest is completed.

In the next chapters I'll describe the different locations and positions

Located behind the Crater House, east-northeast on the shore. The bottle is on the beach between some barrels and a log. Read the message and a waypoint will lead you to the next location. Dive down and loot the steamer trunk inside a sunken ship.

Quick travel to the Castel, turn around and leave the Castel trough the entrance. Go left, around the next corner and when you're facing Spectacle Island you can see the bottle in the sand in front of you. Read the message and a marker will lead you to a submerged boat with a steamer trunk inside. Loot it to complete the objective.

Southwest of the wreck of the FMS Northern Star is an unmarked Lighthouse. In the west of it you'll find the bottle on a sandbar next to a red buoy. Follow the waypiont, dive and open a hatch door in a waste pipe to find the next steamer trunk.

The fourth bottle is located in the southeast of the Boston Airport next to a log, when you're directly facing Fort Strong. After picking up the bottle, dive down to find the wreckage of a Horizon Airlines plane. Inside some stairs leading into the plane’s cockpit, where you'll find the steamer trunk.

Go to east side of the Mahkra Fishpacking. Look for the rowboat with a plastic pumpkin. The bottle message leads you to a nearby steamer trunk on the ground, next to a bathtub.

The last one is located by the Nahant Oceanological Society, on a tiny sand island with picket fencing just off the coast to its southeast. Next to a half-sunken boat. Reaching the marker you have to dive straight to the bottom. As the dive to the sunken boat with the steamer trunk is very deep, it's recommended to unlock waterbreathing via the Aquaboy/Aquagirl perk before.

Maybe this guides are also interesting for you:
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3302154156
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3323162276
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3335971972
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