@bumbumbac
I didn't mean the ship itself, I can well envision what it would be like. I meant, what would these be locked behind? Commendations? Doubloons? Perhaps both. I have a few ideas:
Reaper's sails - a black background, slightly tinged with red, and a large glowing Reaper's Mark icon, possibly torn and replaced with similar off-color fabric akin to the Bone Crusher sails; unlocked by sailing a few hundred nautical miles with the Reaper's Mark up
Reaper's hull - a dark red/near black hull, accented with a bright red and bearing the symbol of the Reaper's Mark on the bow; unlocked by doing several voyages with the Reaper's Mark up (possibly using the Shrouded Spoils or Cursed Crews commendations)
Reaper's figurehead - a statue depicting a robed skeleton with a scythe, accented with a faint glow on the bones and scythe blade, and a strong glow in the eyes; perhaps unlocked by selling reaper's chests?
Alongside these, a capstan and helm unlocked similarly to the sails, and cannons unlocked by shooting enemies (skeletons, pirates, creatures, ships, skeleton ships, etc) with the Reaper's Mark up. A Legendary Reaper's set wouldn't go amiss either, the same commendations with higher numbers and required to be done as a legend. Not sure about equipment or weapons, but clothing could also be done.