This quest is initiated by examining a Salt Encrusted Cutlass. This cutlass is dropped from Captain Rockbelly as part of the X Marks the Spot quest (see related quests).
After examining the cutlass you must take it to Mender Mannus in the Isle of Refuge. He will charge you 40 cp to clean the cutlass. Give it to him and you will receive a version of the weapon that you can use.