The coffin ship is a slow, weak ship with one cargo spot either 1 or 2 item slots. There are two types: Enemy Coffin Ship and the regular Coffin Ship. Enemy Coffin Ships can only be obtained through boarding and capturing during specific quests. These are slower than regular Coffin Ships and only have one item slot. However, they will not be attacked by the AI due to a glitch.
Outside of capturing one, they can very rarely be bought from the Corsairs or received as a reward for NPC quests. Both types of Coffin Ships have no upkeep and have no attack power.
Due to their slow speed, tiny cargo capacity, low health and lack of weaponry, coffin ships aren't useful for anything at all.