Debris are naturally spawning objects in the Shipwrecked DLC. They mark the player's spawn point and are may be meant to infer the player has shipwrecked on their starting island. There are four different pieces: Two planks, a box, and a large broken mast.
Debris can be destroyed with a Hammer to yield 3 Logs and 1 Boat Repair Kit. This allows the player to repair their boats before they gain access to a Science Machine or Stingers, which are required to craft the Boat Repair Kit. Once destroyed, the debris will not respawn.