"Of course, your only reward for this feat is a chest containing some goodies"

first you fing a corpse with a note and the key (which starts the quest to find the treasure), and after you must find chest in the in a small hole nearby.

"Fast-travel back to the “Bay of Winds” signpost on An Skellige (the north-eastern most of Skellige’s islands) and commandeer a ship."

Most likely, both available boats will be near Urialla Harbor.