The Bermuda Triangle Ghosts were made up of the captain and crew of a ship that sunk 50 years before in the Bermuda Triangle.
The ghosts appear as adults from both the ship's crew (in seaman attire) and passengers (in formal eveningwear) who died when the ship was lost. They are partially transparent, unlike other ghosts encountered. They can also appear in a more decayed, horrific form for the purpose of scaring mortals.
The Scooby gang only have interaction with the ship captain, who (while posing as a mortal) is professional, gentle and affable. In ghost form, he seems malevolent and cruel, anxious to exterminate the mortals aboard his ship. The other ship ghosts seem to do his bidding.
The 13 Ghosts of Scooby-Doo
The ghosts lured the gang onto the ghost ship as a cruise vacation, then attacked them in order to free the demons they had recaptured in the Chest of Demons. At least four of them (including the captain) were absorbed into the Cyclops Cyclone Demon in the resulting fray... however there were at least ten of them in the ship's dining hall. The fate of the remaining ghosts is unknown.