I have underwater vampires. In fact, the only undead necropolis in my setting exists underwater. Lack of oxygen also doesn't affect zombies, liches and skeletons, after all, and few can do something against it.
I copied it from a Swamp Thing story from the 80s.
The story went beyond, it had a kind of abyssal-like deep one born from the vampires somehow.
hypnogoria.com/images/reviews/Swamp_Thing_039_09-23-sm.jpg
tomlowell.files.wordpress.com/2009/11/swampthing1.jpg
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Also:
en.wikipedia.org/wiki/Draugr
>Draugar possess superhuman strength, can increase their size at will, and carry the unmistakable stench of decay. "The appearance of a draugr was that of a dead body: swollen, blackened and generally hideous to look at
>The draugr's ability to increase its size also increased its weight, and the body of the draugr was described as being extremely heavy.
>In folklore, draugar slay their victims through various methods including crushing them with their enlarged forms, devouring their flesh, devouring them whole in their enlarged forms, indirectly killing them by driving them mad, and by drinking their blood. Animals feeding near the grave of a draugr may be driven mad by the creature's influence.
>Draugar are noted for having numerous magical abilities (referred to as trollskap) resembling those of living witches and wizards such as shape-shifting, controlling the weather and seeing into the future.
> In legends the hero would often have to wrestle the draugr back to his grave, thereby defeating him, since weapons would do no good.
Sea-draugr would be created when people drowned in the ocean. Just imagine a great viking warrior unjustly tied alive to a ship, cursing his foes and drowning, seeking revenge against family lines generations later through his undead/undersea kingdom.
Drakkars whose rowers never stop for they do not tire, built from whale bones.