The vampire stat block states ([*Monster Manual, p.297*][1]):
>  Vampire Weaknesses.  
The vampire has the following flaws:  
[...]  
*Harmed by Running Water*. The vampire takes 20 acid damage if it ends its turn in running water.

If I were to run a nautically themed Curse of Strahd campaign, could the players simply knock Strahd into the ocean to make this weakness take effect? or is the only running water like a river that has an effect on the vampire?  

My thought is a stream or river's water moves continuously in roughly one direction and roughly constant speed (excluding some seasonal outliers), whereas an ocean's movement is subject to tides and waves, so the movement is not consistent in a direction, and may not be considered 'running' because of this movement.  

Currents in an ocean do tend to move in one direction, but for something the size of a humanoid the wake of a boat, the wind blowing, and creatures moving below the surface are more likely to affect the water around the creature, thus not putting the creature in 'running' water, just moving water, which seems to not count as 'running'.  

If the ocean can be considered running water, would the doldrums be considered standing water? While the ocean as a whole is moving, this one particular area is stationary, is that enough to remove the 'running' tag to this water?

  [1]: https://www.dndbeyond.com/monsters/17043-vampire