This is a two-part custom quest.
It is begun by talking with Finlor the sea captain. He likes to frequent the busiest ports in Britannia. This quest has a thick plot line, and involves a fair bit of globetrotting, but the reward is worth it: a mysterious object which enhances shipboard travel immensely.
The whole quest is not covered here so as not to spoil it for anyone, but if more detailed information is desired, you can head here.