This page in our guide to Greedfall has the walkthrough for The Admiral's Secret Service, one of the side quests.
Where to start the quest?: Speak with the Admiral after completing Treason! main quest.
Talk to captain Ruben, and then speak with his crew. Go to the tavern - inside, you will witness a brawl - tell the thugs to end the conflict. They won't do it - kill them. Approach the person at the bar and talk to him.
The man isn't interested in a chatter - buy a bottle of alcohol and offer it to him. He will become more talkative. The man will show you the place of delivery - go there and stand on the stone ledge nearby. Soon, a suspicious person will come.
Follow the person into town. You reach the culprit - you can use charisma, or threaten him. After finishing the case, return to the Captain to end the quest. You will receive 1,300 XP and +3 Naut reputation.