Very interesting. I studied a little hydrology in undergrad, and my take away was that everything is part of the water cycle. It's rare to have groundwater stand still, and probably from a hydrological perspective there is NO such thing as stagnant water, since it's always evaporating, condensing, being pulled by hydraulic action in some direction...
Anyhow!
I remember in the original Bram Stoker Dracula that Dracula could cross the sea at low tide (or maybe both low and high tide, can't recall exactly). He was drawing on an older folklore about vampires...and the "can't cross running water" limitation seems to have originally been more complex. After all, an ocean surely counts as running water what with tides and currents.
Would be cool, though, if a vampire was trapped by some Aged Druid in an oasis that sprung up overnight, preventing the vampire from ever leaving...