Well the zombies would have to swallow the seawater to be purified wouldnt they?
i seem to recall something stateing in an SBS about rain and waves not weakening DF users.. (and thus shouldnt affect zombies unless they drink it)
oh yeah i even had the page open in another window ^^
http://arlongpark.net/Manga/SBS/41 bottom of the page.
This is different, though. If Nami floods Thriller Bark with salt water, the zombies would be submerged in it. Of course they'd swallow some and be purified. Not to mention that at that point, it would become standing water and DF users would be affected by it.