I'm really starting to like this thing. I've always wanted a Ghost/Water-type anyway.
Ability: either is fine, but I personally like Cursed Body
*Will-o-Wisp or Scald? 'Wisp burns the foe, but Scald has a fair chance of doing that AND deals decent damage. The main point of inflicting burns is to help out Jellicent's lesser Defense. If opting for WoW, go with Hex for its boost against statused opponents; Shadow Ball if Scald instead, in which case you can also add Ice Beam for coverage against Grassers and Dragons. Recover works well with Jellicent's natural bulk, especially if something has been burned.
Other interesting options:
-Water Spout, Energy Ball, Night Shade, Pain Split, Toxic, Trick Room, Hydro Pump, Rain Dance, Wring Out, Sludge Wave.