SOLVED. vampire kidnaps a kid across a river into the multiverse [s]

[Cliff]

I read this one back in middle school, and I didn't get very far. What I remember is that this family is on a trip, and what a vampire absconds with one of the children across a vast river.
I knew that he was a vampire because as they went across the river, he snaps his fingers and a siren or water nymph or some such thing leaps up into the boat they're traveling on. The vampire promptly snaps the poor girl's neck and drinks the resulting blood. He explains that it was his right to the horrified child.

I'm pretty sure this wasn't a vampire novel, really, more of a weird fantasy story.

[Mir]

I have a feeling I read this long ago... maybe something by S.P. Somtow? The The Riverrun Trilogy?

[Cliff]

Yes, that's the one! (I remember the crazy dragon on the the cover now.) Thank you!