Burned on the River

Burned on the River is a side quest in the Fallout 4 Creation Club content "Solar Cannon."

Quick walkthrough

 * First clue at the South Boston military checkpoint on a corpse in a trailer.
 * First safe opened by pressing bell 4 times.
 * Second safe opened by entering code 5-3-6.
 * Third safe opened by code 1-2-11-12.

Detailed walkthrough
Find three notes, one on a Gunner at the South Boston military checkpoint and the others in two safes near Big John's salvage. Use Sarge's Clue #3 ("lining up the cards") to unlock the final safe.

First clue
In a note titled, "Sarge's clue #1" can be found on a Gunner named Jack who's body can be found in a trailer at the military checkpoint. Following the note's instructions and going to the nearby Red Rocket gas station, you can find a wall safe that will open if you activate the bell in the foyer four times. Similarly, you can open the safe by 'hot wiring' it if you have Hacker rank 1, or an Intelligence of 5 or greater.

Second clue
After opening the wall safe in the Red Rocket, you will be given the note titled, "Sarge's clue #2". Heading to a small house not to far to the east of the Red Rocket and going behind it you will find a red dog house, and inside it is a floor safe. The code is 5-3-6, which corresponds to the bowling sign on the road.

Third clue
After popping open the second safe you will receive Sarge's third and final clue, "Sarge's clue #3". Once you find the safe inside a bus inside a nearby underpass, the code is 1-2-11-12, corresponding to the playing card values associated with the names of Sarge's Gunner crew, in the order he listed their names in his third note.

Once the safe is unlocked the quest will immediately complete. Inside the safe is the "Exterminator's Solar cannon rifle" as well as Sarge's note and some other loot.