A derelict is a ship in battle that has been defeated in boarding combat or of which the occupants abandoned ship. A derelict ship can be claimed as a prize by Navy Officers, Freetraders and Privateers to receive a Commendation or a Pennon.
Cutthroats can either capture the ship for their own use through the Take Command of Ship skill or receive varying items using the Scavenge skill, which Buccaneers can also use.
The rewards you get through a derelict are for the player who used the skill and will not go into the combined loot stash for which all players can dice.
A derelict slowly breaks down and eventually sinks unless it is claimed by a player using one of the aforementioned skills.