Another look into a tidbit from vampire mythology had me surfing the net to discover why it is vampires cannot cross running water.
Tradition states running water is pure and cannot hold, or washes away, all magic.
Now there are certain special situations. A vampire may not be able to cross water on his own, but he can be carried over or might change shape and fly or jump over. Dracula was carried across the sea while nestled safely in his coffin on a bed of dirt.
"It is said, too, that he can only pass running water at the slack or the flood of tide." - Professor Van Helsing in Mina Harker's Journal, Chapter XVIII of Dracula by Bram Stoker
"The Count, even if he takes the form of a bat, cannot cross the running water of his own volition, and so he cannot leave the ship." - Professor Van Helsing in Jonathan Harker's Journal, Chapter XXV of Dracula by Bram Stoker
One myth claims burying a vampire on an island prevents him from crossing the water to shore. Also origins point to the myth possibly being about witches instead of vampires and was either borrowed or folded into the vampire myth somehow.
That's about all I found. Anyone seen the myth used in books or movies other than Dracula?