The way I see it is that vampires, being basically corpses, would just sink to the bottom of whatever body of water they wander into, but due to water being water, they have no strength to resist the forces of the water and are treated as if in heavy winds and are pushed around and eventually just die due to the forces of the river tossing them around too violently. So if the water is calm enough, the vampire can traverse them without feat, but something like a running river is like being caught in a tornado for them.

But our group likes to ditch the default vampire rules and borrow stuff from World of Darkness, since we like our vampires to be a bit more complicated, especially in games where they feature as main antagonists or protagonists.