Scribe Bigsley will also give you this one. Go to Rivet City and find Officer Lepelletier. She has an office set up on the ramp leading up to the ship. Start the conversation to learn about the bandit attacks. Go ahead and try the speech challenge. You can get a new plasma rifle if you pass it.
Go over to the marked location under the bridge. The caravan might already be there and previously attacked. The driver is almost always dead. The two security guards may win. If not, then kill the 3 bandits. Either way, search the bandit bodies for a tape explaining that their meeting place is at Wilhelm's Wharf and that the code is Mirelurk Stew. Go to Wilhelm's Wharf to meet the leader, Split Jack.
You have a few options. You can just use a speech challenge and your intelligence to point out that there will never be money to be made from extortion since they aren't selling it in the first place. This isn't too hard and it makes them all walk away.
You can just shoot them. They aren't tough and you won't get bad karma. Go back and report your success.
You can also join the gang. You can then put on the equipment and challenge Split Jack to a dual. You can pick a knife, a pipe, or a nail board for the dual. Don't have any companions with you though, since that counts as cheating. If you win honestly, then the group will split up. This isn't too hard, even for a non-melee player. The only real problem is that you can't hit any of the other bandits. This is much harder than it sounds, since they love to get very close to the action. Stay close to the water to keep them at a safe distance.
You can then go back and extort 500 caps from Lepelletier or demand weekly payments to the gang. She will go hostile if you try to talk to her again. Just put your weapon away and she should stop attacking. You can also just report your success in stopping them. If you go back to Wilhelm's Wharf, then they'll accuse you of backstabbing them and attack.
Go to Scribe Bigsley for your reward. He'll give you some captured Enclave gear for stopping the attacks.
This one can be found in a few ways. You can hack the terminal in Bigsleys office and read the relevant log. You can pass a speech challenge and learn about Griffon's strange purchases in Underworld. You can also hear about it if your karma is already bad. You can also just talk to Griffon at his booth in the entrance to the Museum of History.
Take the free sample and examine it. You will find that it's just dirty water in a new bottle. He's scamming the ghouls into buying dirty water. You can find out a bit more with a speech check and evil karma. Otherwise, you can go back to Scribe Bigsley in the Jefferson Memorial. He'll mention that they drop off the water at the Museum Authority Building.
Go over to the new marked building. It's close to the Museum of History. Go inside and kill a few armed ghouls. Work your way down to the basement and examine the bathtubs. You'll find a few notes in this area and on the patrolling guards detailing his plan. The real stuff is in the tub, waiting to be sold to raiders and slavers, while he's giving the imitation to the ghouls.
Go back to the Museum of History and talk to Griffon. You have three options.
You can demand that he start selling the real stuff to keep your silence. You can come back in a week to get the confirmation. You don't get much out of this option, other than the fact that he really is selling the true stuff.
You can demand money. You can get 300 caps now or a cut of the profits each week.
You can also take the more fun option of demanding that he tells them the truth. He'll reveal the scam and refuse to give them any refunds. The angry crowd then attacks Griffon in a battle through the entrance hall. Watch the fight and loot Griffon's body for 400 caps after they kill him.