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 a quest marker to find a single leveled loot cache.

Locations

 * Bottle message - Trapped for Days is located behind crater house, slightly east-northeast on the shore. Look for some barrels going from crater house to Salem all the way from the shore. It's on the beach between the barrels and a log. Taking the bottle creates a miscellaneous objective on the water surface, swim past the big white and red boat and the wrecked plane. Dive down until one finds a big sewer/pipe and they will see the flipped boat, looting the steamer trunk inside completes the objective.
 * Bottle message - Predator Becomes Prey is located behind The Castle, not far from the south-southeast most star point. It's close to a large round boulder that's sitting right at the water's edge. Only the top of the bottle is visible. Taking the bottle creates a miscellaneous objective on the water surface northwest 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 - Not Going Well 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 - Need a Hand is located at the southeast point of Boston Airport peninsula, on the beach facing Fort Strong between a giant piece of driftwood and three wooden pallets - about 40 yards southeast of the vertibird. 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. The steamer trunk for completing the objective is up the steps inside the plane's cockpit, along with a first aid box.
 * Bottle message - Difficult to Kill is located behind Mahkra Fishpacking, northeast 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 sunken bathtub with a skeleton holding a teddy bear inside and some other items. Also, a steamer trunk with assorted loot (spawned on opening the bottle message) is located in the tub. Taking this loot completes the objective.
 * Bottle message - X Marks the Spot is located by the Nahant Oceanological Society, on a tiny sand island with picket fencing just off the coast to its southeast, just a few paces west of the half-sunken boat. 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 is easy to become lost in the murky water, so waterbreathing via the Aquaboy/Aquagirl perk, a mirelurk cake, or wearing the rescue diver suit is recommended.

Behind the scenes
The fifth bottle message is a direct reference to a line from the 1988 action film Die Hard, sarcastically said by Bruce Willis' character John McClane, in reference to his situation while climbing through a vent in the Nakatomi tower taken over by terrorists.

Bugs
If the second bottle is stored when in workshop mode and later retrieved, this will sometimes cause the note to be 'unreadable'. The quest will appear in your Miscellaneous tab, but there will be no quest marker on the map and the steamer trunk will not spawn, making the quest impossible to complete.