The fishermen, Sonner and Telmen, can be found just south of the Baldur's gate bridge map. Listen to their story then side with them. Speak to Tenya, who can be found northeast of the Baldur's gate bridge, on the bridge map. Return to the fishermen and speak to them. They will confess and off to pay a little more to murder Tenya. Deal with Tenya, then return to the fisherman and tell them.