Vampires are the origin for many of the stories about mermaids and sirens. Long ago, a group of vampires decided to hide from the sun by going deep underwater. They found that this also had the benefit of keeping them safe from vampire hunters. After all, no one questions people going missing at sea and, even if they do, it is hard to attack a vampire lair 100 feet below the water.
Their biggest struggle was that the threshold of a ship still counted as a living space. For some ships, the power of this threshold was even stronger than the average house on land due to the establishment of boarding ceremonies. So, to properly hunt a ship they had to either lure the sailors off the ship or finagle an invite onboard. To do this, the vampires would have their most beautiful women take point and attempt to seduce sailors overboard or get the sailors to haul them on board. If the vampires managed to get onboard and clear a ship, they would sink it or wreck it against a rocky shore to cover their tracks.
Eventually, they started to get more sophisticated with their tactics. During the Golden Age of Piracy, these aquatic vampires found a thriving hunting ground in the Caribbean. They would often take turns having a vampire or two in well-trafficked ports just to serve as scouts. These vampires would attempt to either get invited on board while the ship was in port or at the very least have a solid idea of which ships would be sailing on which routes carrying what cargo. They would then be very careful to not kill anyone in port and would strike the ship when it was isolated at sea. A few vampires who already have an invite onboard clinging to the hull could be a death sentence for a ship.
In other cases, they would pass the information onto pirates who would strike a ship with valuable cargo leaving the crew for the vampires in the water. Some pirates refused to work with vampires, but others saw it as a profitable business venture. In some cases, vampires would keep cargo for themselves and then use their pirate contacts as a way to fence the stolen goods. This has resulted in some of the Caribbean vampires being exceptionally wealthy. It is said that a few underwater caverns are decorated like palaces.
However, around the same time period, the Royal Navy became very well organized in their employment of Wizards. It became exceptionally important for vampires to cover their tracks because while the Crown would sometimes employ pirates as privateers, they were unwilling to enter such an agreement with vampires. Instead, they would do their best to stamp out the presence of any vampires in their territory whether aquatic or traditionally terrestrial.
Some of the aquatic vampire clans were completely wiped out by the English. Careful patrols, scrying, and in some cases simple questioning of witnesses and following evidence revealed the hiding places of the vampires. While hiding deep underwater might have made them unassailable to the average vampire hunter, the Royal Society of Wizards was able to devise methods that allowed even deep caverns to be assaulted. More than one vampire clan found themselves suddenly ambushed by Royal Marines would air bubbles stuck to their heads storming into their “secret” lairs.
However, not all of them were wiped out and some clans adapted to the new normal. A few of them acquired spellcasters of their own (in at least one case turning a Navy Wizard) and began to match Navy magic with magical countermeasures. It allowed these seafaring vampires to remain a presence if not a major force in the Caribbean. When British power began to wane in the Americas centuries later, these vampire clans resurged. In some cases, they were able to prove influential enough to have some puppets in local governments.
A major paradigm shift would come with the aftermath of WWI and the rise of the Romanian Empire. Now with their being a rising world power that was openly (if unofficially at the time) ruled by a vampire, it allowed some faction of vampires to operate more openly. The Caribbean vampires maintained their underwater lairs as their traditional seats of power and at times still raided ships at sea (mostly out of nostalgia). But, they adopted many aspects of the Romanian style of government that allowed vampires to openly rule over their human food source. In the aftermath of WWII, a three-sided Cold War solidified itself into global politics and economics with Romania as the leading nation of one of the factions. Much of the vampire-influenced Caribbean declared alignment with the Romanian faction.