Vampirates #1: Demons of the Ocean book by Justin Somper

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? Published By Little, Brown Books for Young Readers on 2007-04-01