“Have lost heirloom in River Reik. Dropped from Reiksbrücke. Will pay 100 Gold Crowns for retrieval. Contact Jürgen Vasser at the Hangman’s Tavern.”
– A difficult job indeed. Notice posted at several inns along the river Reik.

This little job could open up for splendid opportunities for the players to show their ingenuity. Will they try to find a submersible boat? Or invent a diving bell? Or simply dive from a boat? And what is it that’s been lost? It might indeed be Herr Vasser’s heirloom, but more probably something he has stolen. Maybe from a mage?
So decide what’s been lost, toss this note out to the players and things will roll along nicely from there. Don’t forget that the Niederhafen Pike should play a part in this adventure!