Bottle Message

Bottle messages are both a collection of miscellaneous quests and a series of paper notes in Fallout 4. Collecting each one gives the Sole Survivor quest marker to find a single levelled loot caches.

Locations
Bottle message 1 is located behind Crater House, slightly East-Northeast on the shore. Taking the bottle creates a miscellaneous objective on the water surface, due East. Diving down to the flipped boat and looting the steamer trunk completes the objective.

Bottle message 2 is located behind The Castle, not far from the South-South-East most Star point. Near the water edge, not far from a largish rock standing by istelf near the water. Only the top of the bottle is visible. Taking the bottle creates a miscellaneous objective on the water surface, due East of Castle, and just North West of Spectacle Island. The thin antenna of a submerged boat is just above the surface. Diving down to the boat and taking the loot in the steamer trunk completes the objective.

Bottle message 3 is located near the lighthouse, south of the Wreck of the FMS Northern Star. The bottle can be found on the sand bar with a red buoy, directly west of the lighthouse. After picking up the bottle, a miscellaneous quest marker will appear east-southeast on the surface of the water. Diving below and opening a hatch door in the waste pipe, one can find a first aid kit and a steamer trunk. The objective is completed once the steamer trunk is opened.

Bottle message 4 is located at the south-east point of Boston Airport peninsula, on the beach facing the Fort Strong between a giant driftwood and several wooden planks. After picking up the bottle, a miscellaneous quest marker will appear southeast on the surface of the water. Diving down, the wreckage of a Horizon Airlines plane will be found, taking the loot in the steamer trunk completes the objective.

Bottle message 5 is located behind Mahkra Fishpacking, South East corner, across from the rowboat with the Plastic pumpkin on it. When picked up, it marks a location on the surface of the water near a Buoy East of the packing plant with a miscellaneous objective. Diving deep below that marker is a coffin and a steamer trunk with assorted loot. Taking this loot completes the objective.

Bottle message 6 is located by the Nahant Oceanological Society, on the tiny sand island just off the coast to its south east. The marker for this message is in the middle of a large expanse of water, very roughly south of Croup Manor and Nahant Chapel and roughly east of Nordhagen Beach. The dive to the boat is also very deep, and it easy to become lost in the murky water, waterbreathing via Aquaboy or Mirelurk cake is recommended.

Bottle message 5 - difficult to kill
This is a Die Hard movie reference/easter egg.