Demons of the Ocean (Vampirates Series #1)

Twins, Connor and Grace, never dreamed that there was any truth to the Vampirate shanty their father sang to them before he died, but that was before the two were shipwrecked and separated from each other. For Connor, who is taken aboard a pirate ship, there's the chance to learn to swordfight, but for Grace, aboard a mysterious ship of vampire pirates, the danger is great. What will it take for them to find each other?

"Totally original...I wish I'd had this idea." -Anthony Horowitz